Simplify and optimize getting zip subdirs
Simplify and optimize getting zip subdirs

Since we only care about the first path part, we can stop at 1 split. We
do not need a list, so the unnecessary conversion has been dropped.

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