fix possible issue with transitive build dependencies

[benoit-pierre]

Handle the case where a missing transitive build dependency is required by an extra for an already installed build dependency.

Example:

• a test package with a setup.py containing: setup_requires=['dep[extra]']
• dep is already installed, and the dependencies for extra are: ['extra_dep']
• extra_dep is not already installed, and fetch_build_egg('extra_dep; extra =="extra"') is called

If the marker is not stripped, pip wheel [...] 'extra_dep; extra =="extra"' ignores the requirement, and does not build a wheel, but still returns with no error.

Summary of changes

Closes the second issue found in #1920.

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d. See documentation for details

[frivoire]

It seems I'm impacted by this bug too, with one difference : my use case is dependencies in tests_require and not in setup_requires.

So, the proposed fix should be ok (it's in a generic code path that is used for both tests_require and setup_requires as far as I understand) :)

EDIT: I tested the patch of this PR in my context, and it does indeed fix the IndexError :)