This is a rewrite of commit 6e4b8efe
("Support network cache in Download.download()").

Like for URL that contain credentials, we still skip auth challenge
because it's faster and:
- we only support one auth scheme (basic)
- netrc provides no way to specify realms, which seem anyway to be
  less and less used (https://stackoverflow.com/q/69303610 reports
  that recent browsers don't display them anymore)

See merge request nexedi/slapos.buildout!25

- Py3: stop using legacy API of urllib.request and
       fix download of http(s) URLs containing user:passwd@
- Py2: avoid OOM when downloading huge files

This is implemented as a method in case we want to make it configurable
via [buildout].

To be dropped once all buildout in the wild are able to upgrade to a
version that supports new names in expression of conditional sections
(see previous commit).

Adding new names for expression is currently not possible because
buildout aborts before it tries to upgrade (in-place or bootstrap).

(cherry picked from commit f1589378)

For example, existing values were not enough to distinguish
'arm-linux-gnueabi' from 'arm-linux-gnueabihf'.

The 'multiarch' value is the output of

  $CC -dumpmachine

where CC defaults to 'gcc'.

See also https://wiki.debian.org/Multiarch/Tuples

Sometimes, most parts are reinstalled for a reason that the user
didn't think about and it can take time to understand why.
Explaining for all parts would be too verbose and useless because
many are reinstalled just because their dependencies changed.

This alternate URL is used in case of HTTPError with the main one.
In any case, the main URL is used for both downloading & uploading
from/to networkcache.

This will be exposed by slapos.recipe.build:download* and
slapos.recipe.cmmi recipes.

An optimization is to avoid temporary file when possible: a rename
(or hard link) is not always possible (different mount points).

Another one is to not check md5sum twice when using cache file.

Fall-back mode is ignored if an MD5 checksum is given.

In case of checksum mismatch for a cached path, remove it and
download again, mainly to cover the following cases:
- the url content changes and the user updates the checksum
- buildout killed while downloading directly to cache
  (see above optimization)
- shutil.copyfile is interrupted

Egg is install from wheel the requested version ends with :whl

This replaces:
	<removed>

When recipes mutate the options, we should not allow invalid syntax otherwise
it gets written in .installed.cfg and generate invalid configparser syntax.

When recipe mutate options, options values are written to .installed.cfg
without escaping buildout substitution syntax, so if a recipe sets an option
value that could be interpreted as a buildout substitution, it is written as is
in .installed.cfg.
This can be a problem if options read from _read_installed_part_options are
accessed, like it's the case with slapos patched buildout which saves
installed options after an error with part installation or after each part
installation when running in verbose mode.

python ZipFile module does not support updating an entry in place, instead make
a new zip file and copy all entries.

On Python 3, these made the build output very hard to read (e.g., https://travis-ci.org/buildout/buildout/jobs/394026829)

On Python 3.7, these could actually break the doctests.

Closing the files should get us closer to being able to pass the tests with PyPy.

Add a RENormalizer so the easy_install.txt will be happy with scripts
generated on Python 2 (using open(..., 'U')) and on Python 3 (using just
open(...)).

This was suggested by @sallner.

(cherry picked from commit 2cf42066)

Python 3.7 began warning about open's 'U' mode. Universal newline mode (newline=None) is default in Python 3 so avoid specifying it on Python 3.

(cherry picked from commit 978a6af9)

fixup! Apply += & -= in correct order with respect to extends

If a.cfg extends b.cfg and both contain [a],
it was already not possible for a.cfg:[a] to combine <= & +=.
But it worked when a.cfg:[a] does not extend anything, and
commit 161fb191 broke that.

The only proper way to solve all this seems to process <= before
+= & -=. But since processing <= while extending files would cause
other issues, we'd rather process +=&-= when initializing parts.

This is useful when using OS Python & eggs.

Useless for SlapOS.

The version of Python should not affect the behaviour of a recipe.
Anyway, it was already ignored for DEVELOP_DIST eggs.

This makes the slapos mechanism to share parts more efficient. And with
the upcoming changes in buildout & slapos.recipe.cmmi, there would be
no way for the slapos.reboostrap extension to prevent everything from
being rebuilt when reboostrapping to a different version of Python,
if bootstrap parts are shared.

The monkey-patch by slapos.reboostrap now becomes useless.

When buildout encounter an error, program exists with error code 1, this
should also be the case when exiting from a debugger session when
running with -D option.

/reviewed-on nexedi/slapos.buildout!17

When a recipe is developed in a git working copy, .git administrative
database folder should be ignored, like we did with CVS and svn.

/reviewed-on nexedi/slapos.buildout!14

_install_and_load is slow, using cache saves time when there are many sections.

Contrary to bash, dash filters out environment variables containing
'-' characters. And for example, /bin/sh is dash on Debian.

<rebased>

Useful when recipes generate non-string values to be reused by other
recipes.