Made .module.install staging rules more clear

There are four cases of $src and $dst:

    foo   bar   # stages file     foo   to bar
    foo   bar/  # stages file/dir foo   to bar/foo
    foo/  bar   # stages      dir foo/. to bar/.
    foo/  bar/  # stages      dir foo/. to bar/foo/.

and some more with dots and stars:

    foo/. bar   # stages      dir foo/. to bar/.
    foo/. bar/  # stages      dir foo/. to bar/foo/.
    foo/* bar/  # stages files in foo/* to bar/*