Add function to install directly from wheel files
Add function to install directly from wheel files

This will help us avoid some complicated directory-changing logic in
WheelBuilder.

xavfernandez/pip,pfmoore/pip,xavfernandez/pip,xavfernandez/pip,sbidoul/pip,pypa/pip,sbidoul/pip,pypa/pip,pfmoore/pip,pradyunsg/pip,pradyunsg/pip