feat(pypi): pip.defaults API for customizing pipstar 1/n

[aignas]

Parse env markers in pip.parse using starlark

Summary:

• Allow switching to the Starlark implementation of the marker evaluation function.
• Add a way for users to modify the env for the marker evaluation when parsing the requirements. This can only be done by rules_python or the root module.
• Limit the platform selection when parsing the requirements files.

Work towards #2747 Work towards #2949 Split out from #2909