Support extras that reference other extras

I want to do something like this (simplified for the example):

setup(
    name='my-project',
    extras_require={
        'tester': [
            "pytest==3.3.2",
        ],
        'dev': [
            "pytest-xdist",
            ".[tester]",
        ],
    },
    ...
)

When I run pip install -e .[dev], I want it to install pytest and xdist. Right now, it fails saying that I supplied an invalid package name.

Also of interest is that the following incantation spits no error, but seems to ignore the tester extra:

        'dev': [
            "pytest-xdist",
            "my-project[tester]",
        ],

I’m open to attempting at a PR, but was curious if someone had a reason that I shouldn’t even attempt this.

[benoit-pierre]

@skirpichev: actually, you can rely on the fact that interpolation is enabled:

[options.extras_require]
tester =
    pytest==3.3.2
    pytest-sugar
dev =
    pytest-xdist
    %(tester)s

[benoit-pierre]

It was considered a bug, introduced with Python 3, but it was found that the behaviour has been present all along (with Python 2 too), so disabling interpolation would be a backward incompatible change. Allowing the use case above is a good argument for keeping it, so maybe we should just make it official (mention it in the documentation, and add a test for it). @jaraco, @pganssle: thoughts?