A recent change in pip wheel cache behavior had upstream pip indicating
that we really should be using pip wheel instead. The reason we weren't
using pip wheel appears to be that we wanted to infer what top level
wheel to install via contents of a dir separate from our wheel output
dir/wheel cache. Using pip wheel implies everything gets flattened into
one location. We deal with this by having the build tool write all of
the top level wheels we care about into a separate location. Later we
can install all of the top level wheels while pointing find links at the
larger set of deps in the dir created by pip wheel.
Change-Id: Id9c674c1ec6fe5e72534549082e3adda9e286fd5
521df0eda0