[Distutils] Installed Extras Metadata

[Pradyun Gedam]

Hello! I hope everyone's had a great start to 2018! :)

A few months back, while working on pip, I had noticed an oddity about
extras.

Installing a package with extras would not store information about the fact
that the extras were requested. This means, later, it is not possible to
know which extra-based optional dependencies of a package have to be
considered when verifying that the packages are compatible with each other.
This information is relavant for resolution/validation since without it, it
is not possible to know which the extra-requirements to care about.

As an example, installing ``requests[security]`` and then uninstalling
``PyOpenSSL`` leaves you in a state where you don't really satisfy what was
asked for but there's no way to detect that either.

Thus, obviously, I'm interested in making pip to be able to store this
information. As I understand, this is done needs to be specified in a PEP
and/or on PyPUG's specification page.

To that end, here's seeding proposal for the discussion: a new
`extras-requested.txt` file in the .dist-info directory, storing the extra
names in a one-per-line format.

Cheers!
Pradyun
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