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)
OK, I took a stab at this, because that stuff is used in a lot of places, this change would end up touching a lot of files.

See https://gist.github.com/msabramo/7b9696d69d870a9878a3

I am thinking that might be too big of a change to entertain if you folks want to release pip 6 soon?

If not, let me know and I can add it to this PR; otherwise, I think I would recommend merging this PR as-is and filing an issue to do the refactor later. What do you think?