Support PEP 345 Project-URL metadata

Add setup.cfg support for long_description_content_type

Fix trailing slash handling in pkg_resources.ZipProvider

Given a ZipProvider, if the underlying ZipImporter prefix is empty,
then resource_listdir('') and resource_listdir('subdir/') fail, while
resource_listdir('/') and resource_listdir('subdir') succeed.

On the other hand, if the underlying ZipImport prefix is not empty,
then resource_listdir('/') fails but resource_listdir('') succeeds.

With this change, the cases listed succeed with or without trailing
slashes.

Add an entry to the Setuptools usage documentation for the
project_urls dict, and include it in the "advanced" project example.

While at it, adjust the dogfooding use in Setuptools' own setup.py
to use HTTPS for the Documentation URL and drop redundant entries
for Bug Tracker and Source Code.

Also remove a no-op attribute reassignment from egg_info.py.

fix `data_files` handling when installing from wheel

fix namespace packages handling of wheels

easy_install: add support for installing from wheels

Note: wheels are installed as eggs, so each install is self-contained
and multiple versions of the same package can be installed at the same
time.

Limitations:
- headers are not supported
- resulting egg metadata requirements have their markers stripped

Fixes #1208: Improved exception message of pkg_resources.ResolutionError

Add another test demonstrating that if requirements are declared in a non-deterministic order, they may appear in the metadata in non-deterministic order. Ref #458.