Handle extras in entry_points.txt

[aignas]

This is split from #1363 as discussed in #1363 (comment)

See https://packaging.python.org/en/latest/specifications/entry-points/#data-model

[aignas]

I guess the relative priority of this feature is low given that the usage of extras for the entry_points is no longer recommended (see here):

Using extras for an entry point is no longer recommended. Consumers should support parsing them from existing distributions, but may then ignore them. New publishing tools need not support specifying extras. The functionality of handling extras was tied to setuptools’ model of managing ‘egg’ packages, but newer tools such as pip and virtualenv use a different model.

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[aignas]

Not sure if there is a need to do this. Users who need this can request a package with extras in the requirements file and then use the entry points rule to get the entrypoint.