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)
Merge pull request #2183 from msabramo/display_url_downloading_from

Display URL downloading from