req: fix possible exception in install_req_from_req

[benoit-pierre]

• allow calling it with comes_from=None (easier for testing)
• handle the case where comes_from.link is None (which can happen if a package using a PEP 508 URL requirement is already installed)

Fix #5889.

Note: this depends on #5892 for the test.

[cjerdonek]

@benoit-pierre Is there a reason the fix for #5889 has to be combined with other code? Is it possible to submit a PR just with the comes_from check and some simple unit tests of install_req_from_req()? It seems that would make the patch smaller and more focused and easier to review, and the test more specific to the particular issue, etc.

[benoit-pierre]

Yes, as mentioned in the note I need #5892 for the test: pkg @ file:///path/to/wheel cannot be used because of pypa/packaging#120, and pkg @ file://localhost/path/to/wheel does not without #5892 on Linux because pip tries to use a UNC path...

[cjerdonek]

What about a simple unit test of install_req_from_req() that exercises that code path? I don't see one in the PR. I only see a higher-level end-to-end test.

[lock]

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