Fix environment markers handling

[benoit-pierre]

While the documentation now points to using environment markers with install_requires for declaring platform dependencies, this result in invalid wheels because those are stripped from the final requirements in the embedded metadata.

E.g, using something like this:

from setuptools import setup
setup(
    name='foo',
    install_requires='barbazquux; "foobar" in sys_platform',
)

result in foo.egg-info/requires.txt containing:

barbazquux; "foobar" in sys_platform

instead of:

import sys

from setuptools import setup

setup(
    name='foo',
    extras_require={':"foobar" in sys_platform': 'barbazquux'},
)

which yields:

[:"foobar" in sys_platform]
barbazquux

This PR automatically move requirements in install_requires that use environment markers to extras_require so the egg info is valid and the generated wheels work as expected.

Additionally, I added a check to error out if an extras_require requirement is itself using an environment marker, as it's not supported.

Note: first commit is a fix for the same issue as #1063, so I'll rebase once this is merged.

[benoit-pierre]

Rebased on master.