don't install empty dirs during wheel installs

a fix for #1632, that solves the problem by not installing empty directories (including the empty `__pycache__` dirs in pypy under dist-info).  this should really only be an issue an odd cases, as packaging doesn't allow/create empty dirs afaik

note that I'm wanting to start a trend here of commiting the src for test packages (when it's something we've constructed) to tests/data/src, and in general use src for project src, and leave packages just for packages.