Alternative to #2375

When other libraries or tools add items to the meta_path, we need to preempt
some of their import hooks to be sure modules can be properly found. This also
prevents problems importing built-in modules on Python 2 where it will first
attempt to import something like:

    requests.packages.chardet.sys

By placing our VendorAlias first, the above will fail and then it will fall
back to trying to import sys directly instead.

Closes #2465

When working these changes back upstream to pip, we realized that the
previous fix wasn't ideal since unvendoring the packages broke the
imports. For example, if urllib3 were unvendored, then the following
would fail:

    from requests.packages import urllib3

While discussion the issue, Donald Stufft (@dstufft) and I realized the
simplest solution is to simply add an alias per vendored dependency. The
resulting changes are simple and effective. It prevents the issue in
2.5.2 and 2.5.3 where the following would work:

    from requests.packages import webbrowser

This now appropriately raises an ImportError.

Closes #2465