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>
(cherry picked from commit ad44e47542a822ac3e02cf514b5d2be52880fc95)
Correctly clone submodules from github

(cherry picked from commit b755c7bd6af9f2bee47427b1eaa6c29c72b2b17a)
Make sync-all handle all github protocols correctly

This fixes #8824.

(cherry picked from commit 3efcb0a7d147e05f86501783144bcd0ad3757e93)
Also allow http://github.com (#8824)

(cherry picked from commit d246c62afd7312185aee9433b065ea99e4fa4054)