Switch to relative URLs in .gitmodules

Previously, the `http://`-protocol part was hardcoded in the URLs, causing
the initial clone process to fall back to `http://` even when the ghc.git repo
was cloned via one of the other 3 supported transport protocols.

This is slightly related to #8545, as it will make it possible to e.g.

    git clone --recursive git://git.haskell.org/ghc

and clone ghc.git including all submodules in one go (i.e. w/o `sync-all`),
and w/o falling back to a different (hardwired) Git transport protocol for
the submodules.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>