Make `Setup.hs copy/install` work when data-files uses **. (#6127)

Treating globs like filenames was always illegitimate, but this code
was broken further by the addition of recursive globs.

I had a look around for other dubious code along these lines, and it
looks like this site is the only problematic one.

Fixes #6125.

(cherry picked from commit 7fec503a)