Apply packaging.tags patch to pip's vendored version

[virtuald]

• Fixes pip ignores --platform-tags #126

Would love a release with this. We use a custom platform tag for our wheels so they only install on our controller, but we also want to be able to leverage normal wheels for our architecture too. This allows that to happen.

[virtuald]

I noticed my initial patch actually breaks existing behavior... but I think it's the correct fix, but probably should be documented?

With this patch, if no platform tags are reported, pip debug reports:

Compatible tags: 45
  cp313-cp313-armv7l
  cp313-abi3-armv7l
  ...

Previously, pip debug would have reported:

Compatible tags: 45
  cp313-cp313-linux_armv7l
  cp313-abi3-linux_armv7l
  ...

I think this is because the unpatched version of packaging.tags (https://github.com/pypa/packaging/blob/0055d4b8ff353455f0617690e609bc68a1f9ade2/src/packaging/tags.py#L578) checks sysconfig.get_platform() and if it contains linux then it yields linux_{arch}.

I think the correct behavior is that if a user specifies --platform-tag, then they must declare all compatible tags. Otherwise, we should default to linux_{arch}.