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 Trac #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
Signed-off-by: Herbert Valerio Riedel <email@example.com>