`makeWrapper` and `wrapProgram` are being invoked on all kinds of
wacky things (usually with the help of bash globs or other machine
assistance).
So far, I have come across `wrapProgram` being invoked on a directory,
as well as on the empty string.
As far as I can tell, it's only valid to invoke these utilities on a
normal (non-directory, non-device) executable file. This commit
enforces that precondition.
e1d46c0c4d