Fix prefix bug in Haskell ssconf implementation

This patch fixes a (brown-bag) bug in the ssconf implementation where
the looked-for filenames were not prefixed with the ssconf file
prefix.

Additionally, a test for this is added (which fails without the fix).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>