[Distutils] distlib updated - comments sought

[Vinay Sajip]

I've updated distlib as follows:

1. I've added support for the equivalent of "pkg_resources.resource_filename"
   for returning a true filesystem path for a resource. I've added info about
   this in the "Distlib's design" document, though the reference API has not
   been fully updated - I thought I'd see if there was any feedback about the
   approach I've used. Code and tests have been added, though.

2. I've added the PyPI client code from distutils2 in distlib.pypi. I only
   tweaked it a little to run under 2.x and 3.x from a single code base,
   ensuring that all the tests passed. I hope to update it with better support
   for finding archives not hosted on PyPI, using hints.

I'm also considering the following changes, which I'd like some feedback on:

* Merging the distlib.depgraph package into the distlib.database package, as
  they are fairly interlinked (e.g. adding requires() and required_by() methods
  to Distribution).

* Removing depgraph.main() and its test, because I don't think it belongs in the
  library layer, but is better implemented as a utility script.

Please let me have your comments.

Regards,

Vinay Sajip