Parse requirements with environment markers

[gforcada]

For example on Products.MimetypesRegistry we have:

'pywin32 ; platform_system=="Windows"',

Which actually provides different imports, namely win32api and win32con.

You can do a mapping for them:

'pywin32' = ['win32api', 'win32con']

But z3c.depenedencychecker still fails with it 😓

When you remove the environment marker though, then it all works 🎉

Whenever we are parsing the requirements of a given distribution we are reading the file and being happy with it. We should parse those lines and remove the environment markers.