Migrate to importlib-metadata

[uranusjr]

Progress (probably not fix) #7413.

The plan is to implement a common interface in pip._internal.metadata, and gradually move direct pkg_resources.Distribution to go through it.

[pradyunsg]

Any specific reason to not directly assign this, outside of the if?

[pradyunsg]

This should be automatically handled by vendoring -- does it not do this?

[pradyunsg]

Any specific reasons to do this?

[uranusjr]

The version function looks for the importlib-metadata distribution itself in sys.path, and breaks with vendoring since there’s no metadata anywhere to be found. I figure it’s easier to remove the global call (which fails on import time) since we have no interest in reading __version__ anyway.

[pradyunsg]

A comment describing this somewhere on the patch, would be useful. Instead of removing the lines, consider commenting them out and noting why, similar to how we do it in requests.patch for the simplejson import. :)

[xavfernandez]

Yup, as reminded in #7784 patches should be dcumented/motivated here: https://github.com/pypa/pip/blob/master/src/pip/_vendor/README.rst#modifications

[xavfernandez]

And concerning the version value, it might actually be useful if we wanted to include the vendored libraries' version in pip debug output to help diagnose tweaked installation. But that's not currently the case indeed.