makeWrapper: Only wrap normal executable files

`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.