Reduce duplication of "pypi.python.org"

by creating a `PyPI` object (of class `Index`)

Inspired by [comment](https://github.com/pypa/pip/pull/2183#issuecomment-66967168) from @xavfernandez:

> I'm surprised "pypi.python.org" is not a constant defined somewhere in pip's code, just like distlib seems to do

Before:

    $ ag --ignore=_vendor 'pypi.python.org' pip
    pip/cmdoptions.py
    195:    default='https://pypi.python.org/simple/',

    pip/commands/search.py
    33:            default='https://pypi.python.org/pypi',

    pip/index.py
    305:            if page is None and 'pypi.python.org' not in str(main_index_url):
    706:                        ).netloc.endswith("pypi.python.org")):

    pip/utils/outdated.py
    107:                "https://pypi.python.org/pypi/pip/json",

After:

    $ ag --ignore=_vendor 'pypi.python.org' pip
    pip/index.py
    77:PyPI = Index(url='https://pypi.python.org/', trusted=True)
OK, updated to use `urllib_parse.urljoin`.