memoize calls to pkg_resources.build_zipmanifest

This addresses
https://bitbucket.org/pypa/setuptools/issue/154/build_zipmanifest-results-should-be

We have some .pex files with 30+ egg dependencies.  They now take 5-10s just
to start.  This drops their startup time back to pre-regression levels.

Testing Done:
mba=twitter-commons=; build-support/bin/ci.sh
...

== CI SUCCESS ==

Reviewed at https://rbcommons.com/s/twitter/r/53/