Add -markerlib and pkg-resources to the list of PACKAGES_TO_IGNORE

[cworth-gh]

On Debian unstable, mkvirtualenv 15.0.0 is installing two pseudo
packages (each with a version number of 0.0.0) as can be seen here:

$ mkvirtualenv foo
Running virtualenv with interpreter /usr/bin/python2
New python executable in /home/cworth/.virtualenvs/foo/bin/python2
Also creating executable in /home/cworth/.virtualenvs/foo/bin/python
Installing setuptools, pkg_resources, _markerlib, pip, wheel...done.

But then pip-sync will try to uninstall these packages (since nothing
explicitly depends on them) which will then cause pip stop working as
can be seen here:

$ pip-sync reqs.txt
Uninstalling pkg-resources-0.0.0:
  Successfully uninstalled pkg-resources-0.0.0

$ pip-sync reqs.txt
Traceback (most recent call last):
  File "/home/cworth/.virtualenvs/foo/bin/pip-sync", line 5, in <module>
    from pkg_resources import load_entry_point
ImportError: No module named pkg_resources

This commit adds these two package names to the PACKAGES_TO_IGNORE
list so that pip-sync will not try to uninstall them.

[tekacs]

Currently having to fix this manually, everywhere - would be great to see this merged!

[palfrey]

I'd also like to see this in. Any thoughts on getting it merged? Is it worth setting up a separate PR with just the sync.py changes?

[ericamador]

I know this repository recently changed ownership, so I'm bumping this to see if anything needs to be changed in to get the first commit of this series merged?

[vphilippon]

@amadornimbis It definitely needs a rebase on master. The version update in setup.py and CHANGELOG.md should be removed too, as that'll be handled when doing the release (the CHANGELOG entry itself should be kept though).

pkg_resources seems fine to add to the list, as it usually comes from setuptools anyway.
_markerlib is not a valid package name to install anyway (pip install _markerlib leads to and InvalidRequirement error), so I feel like it can be ignored too.

@davidovich Could you take a look at this? It seems like this would fix some error related to pip-sync uninstalling pkg_resources.

[oxplot]

As a workaround, you can setup the env with no setuptools and install it after activation:

virtualenv --no-setuptools venv
. venv/bin/activate
pip install setuptools
pip-sync requirements.txt

[vphilippon]

Will be completed in #613

[graingert]

instead of ignoring -markerlib what about also ignoring anything without a valid package name:

^([A-Z0-9]|[A-Z0-9][A-Z0-9._-]*[A-Z0-9])$

[graingert]

@vphilippon as defined: https://github.com/pypa/warehouse/blob/2b54d1e6a004ced695745d1f38a4304d81d6f8f7/warehouse/packaging/models.py#L82

[vphilippon]

@graingert Hm, maybe. but that should be opened in its own issue, that'll be lost on here.

[graingert]

@vphilippon #618 done