docs: dedicate file for package discovery

docs/userguide/package_discovery.txt

+===================
+Package Discovery
+===================
+
+
+
+Using ``find_packages()``
+-------------------------
+
+For simple projects, it's usually easy enough to manually add packages to
+the ``packages`` argument of ``setup()``.  However, for very large projects
+(Twisted, PEAK, Zope, Chandler, etc.), it can be a big burden to keep the
+package list updated.  That's what ``setuptools.find_packages()`` is for.
+
+``find_packages()`` takes a source directory and two lists of package name
+patterns to exclude and include.  If omitted, the source directory defaults to
+the same
+directory as the setup script.  Some projects use a ``src`` or ``lib``
+directory as the root of their source tree, and those projects would of course
+use ``"src"`` or ``"lib"`` as the first argument to ``find_packages()``.  (And
+such projects also need something like ``package_dir={"": "src"}`` in their
+``setup()`` arguments, but that's just a normal distutils thing.)
+
+Anyway, ``find_packages()`` walks the target directory, filtering by inclusion
+patterns, and finds Python packages (any directory). Packages are only
+recognized if they include an ``__init__.py`` file. Finally, exclusion
+patterns are applied to remove matching packages.
+
+Inclusion and exclusion patterns are package names, optionally including
+wildcards.  For
+example, ``find_packages(exclude=["*.tests"])`` will exclude all packages whose
+last name part is ``tests``.   Or, ``find_packages(exclude=["*.tests",
+"*.tests.*"])`` will also exclude any subpackages of packages named ``tests``,
+but it still won't exclude a top-level ``tests`` package or the children
+thereof.  In fact, if you really want no ``tests`` packages at all, you'll need
+something like this::
+
+    find_packages(exclude=["*.tests", "*.tests.*", "tests.*", "tests"])
+
+in order to cover all the bases.  Really, the exclusion patterns are intended
+to cover simpler use cases than this, like excluding a single, specified
+package and its subpackages.
+
+Regardless of the parameters, the ``find_packages()``
+function returns a list of package names suitable for use as the ``packages``
+argument to ``setup()``, and so is usually the easiest way to set that
+argument in your setup script.  Especially since it frees you from having to
+remember to modify your setup script whenever your project grows additional
+top-level packages or subpackages.
+
+``find_namespace_packages()``
+-----------------------------
+In Python 3.3+, ``setuptools`` also provides the ``find_namespace_packages`` variant
+of ``find_packages``, which has the same function signature as
+``find_packages``, but works with `PEP 420`_ compliant implicit namespace
+packages. Here is a minimal setup script using ``find_namespace_packages``::
+
+    from setuptools import setup, find_namespace_packages
+    setup(
+        name="HelloWorld",
+        version="0.1",
+        packages=find_namespace_packages(),
+    )
+
+
+Keep in mind that according to PEP 420, you may have to either re-organize your
+codebase a bit or define a few exclusions, as the definition of an implicit
+namespace package is quite lenient, so for a project organized like so::
+
+
+    ├── namespace
+    │   └── mypackage
+    │       ├── __init__.py
+    │       └── mod1.py
+    ├── setup.py
+    └── tests
+        └── test_mod1.py
+
+A naive ``find_namespace_packages()`` would install both ``namespace.mypackage`` and a
+top-level package called ``tests``! One way to avoid this problem is to use the
+``include`` keyword to whitelist the packages to include, like so::
+
+    from setuptools import setup, find_namespace_packages
+
+    setup(
+        name="namespace.mypackage",
+        version="0.1",
+        packages=find_namespace_packages(include=["namespace.*"])
+    )
+
+Another option is to use the "src" layout, where all package code is placed in
+the ``src`` directory, like so::
+
+
+    ├── setup.py
+    ├── src
+    │   └── namespace
+    │       └── mypackage
+    │           ├── __init__.py
+    │           └── mod1.py
+    └── tests
+        └── test_mod1.py
+
+With this layout, the package directory is specified as ``src``, as such::
+
+    setup(name="namespace.mypackage",
+          version="0.1",
+          package_dir={"": "src"},
+          packages=find_namespace_packages(where="src"))
+
+.. _PEP 420: https://www.python.org/dev/peps/pep-0420/
+
+
+Namespace Packages
+------------------
+
+Sometimes, a large package is more useful if distributed as a collection of
+smaller eggs.  However, Python does not normally allow the contents of a
+package to be retrieved from more than one location.  "Namespace packages"
+are a solution for this problem.  When you declare a package to be a namespace
+package, it means that the package has no meaningful contents in its
+``__init__.py``, and that it is merely a container for modules and subpackages.
+
+The ``pkg_resources`` runtime will then automatically ensure that the contents
+of namespace packages that are spread over multiple eggs or directories are
+combined into a single "virtual" package.
+
+The ``namespace_packages`` argument to ``setup()`` lets you declare your
+project's namespace packages, so that they will be included in your project's
+metadata.  The argument should list the namespace packages that the egg
+participates in.  For example, the ZopeInterface project might do this::
+
+    setup(
+        # ...
+        namespace_packages=["zope"]
+    )
+
+because it contains a ``zope.interface`` package that lives in the ``zope``
+namespace package.  Similarly, a project for a standalone ``zope.publisher``
+would also declare the ``zope`` namespace package.  When these projects are
+installed and used, Python will see them both as part of a "virtual" ``zope``
+package, even though they will be installed in different locations.
+
+Namespace packages don't have to be top-level packages.  For example, Zope 3's
+``zope.app`` package is a namespace package, and in the future PEAK's
+``peak.util`` package will be too.
+
+Note, by the way, that your project's source tree must include the namespace
+packages' ``__init__.py`` files (and the ``__init__.py`` of any parent
+packages), in a normal Python package layout.  These ``__init__.py`` files
+*must* contain the line::
+
+    __import__("pkg_resources").declare_namespace(__name__)
+
+This code ensures that the namespace package machinery is operating and that
+the current package is registered as a namespace package.
+
+You must NOT include any other code and data in a namespace package's
+``__init__.py``.  Even though it may appear to work during development, or when
+projects are installed as ``.egg`` files, it will not work when the projects
+are installed using "system" packaging tools -- in such cases the
+``__init__.py`` files will not be installed, let alone executed.
+
+You must include the ``declare_namespace()``  line in the ``__init__.py`` of
+*every* project that has contents for the namespace package in question, in
+order to ensure that the namespace will be declared regardless of which
+project's copy of ``__init__.py`` is loaded first.  If the first loaded
+``__init__.py`` doesn't declare it, it will never *be* declared, because no
+other copies will ever be loaded!
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