Display URL downloading from

Display URL downloading from instead of just filename when using index other than PyPI.

It's useful to distinguish between downloading from PyPI or from an internal devpi server, for example. In the latter case, it is useful to see the full URL, to know which index pip is downloading from.

E.g.:

Downloading from PyPI is unchanged:

    $ pip install --no-cache-dir --ignore-installed Jinja2
    ...
      Downloading Jinja2-2.7.3.tar.gz (378kB)

But downloading from a different server results in displaying the full URL:

    $ pip install --no-cache-dir --ignore-installed -i http://mirror.picosecond.org/pypi/simple jinja2
    ...
      Downloading http://mirror.picosecond.org/pypi/packages/source/J/Jinja2/Jinja2-2.7.3.tar.gz (378kB)
Ah, cool. Thanks for clarifying!

There's a wrinkle -- `pip.index` depends on `pip.download`; if I add `from pip.index import PyPI` to the top of `pip/download.py`, that creates a circular dependency. I could add a dynamic import, but that's gross and I'd rather take the circular import smell as a design smell and figure out a better structure... but I don't know what that is yet... Ideas?