Resolve special strings in URL to full URL

[matthewrsj]

In options.conf['package']['url'], resolve strings such as 'pypi://' to either pypi.debian.net or pypi.python.org depending on which is available (pypi.debian.net is often down, but offers a much easier URL scheme). This could be determined easily with pip download -v package and parsing the output to determine the URL to put in the spec file.

This could make future mirror:// resolution possible as well for the more general cases.