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 ad44e475)

.gitmodules

 [submodule "libraries/binary"]
 	path = libraries/binary
-	url = http://git.haskell.org/packages/binary.git
+	url = ../packages/binary.git
 	ignore = untracked
 [submodule "libraries/bytestring"]
 	path = libraries/bytestring
-	url = http://git.haskell.org/packages/bytestring.git
+	url = ../packages/bytestring.git
 	ignore = untracked
 [submodule "libraries/Cabal"]
 	path = libraries/Cabal
-	url = http://git.haskell.org/packages/Cabal.git
+	url = ../packages/Cabal.git
 	ignore = untracked
 [submodule "libraries/containers"]
 	path = libraries/containers
-	url = http://git.haskell.org/packages/containers.git
+	url = ../packages/containers.git
 	ignore = untracked
 [submodule "libraries/haskeline"]
 	path = libraries/haskeline
-	url = http://git.haskell.org/packages/haskeline.git
+	url = ../packages/haskeline.git
 	ignore = untracked
 [submodule "libraries/pretty"]
 	path = libraries/pretty
-	url = http://git.haskell.org/packages/pretty.git
+	url = ../packages/pretty.git
 	ignore = untracked
 [submodule "libraries/terminfo"]
 	path = libraries/terminfo
-	url = http://git.haskell.org/packages/terminfo.git
+	url = ../packages/terminfo.git
 	ignore = untracked
 [submodule "libraries/transformers"]
 	path = libraries/transformers
-	url = http://git.haskell.org/packages/transformers.git
+	url = ../packages/transformers.git
 	ignore = untracked
 [submodule "libraries/xhtml"]
 	path = libraries/xhtml
-	url = http://git.haskell.org/packages/xhtml.git
+	url = ../packages/xhtml.git
 	ignore = untracked
 [submodule "libraries/Win32"]
 	path = libraries/Win32
-	url = http://git.haskell.org/packages/Win32.git
+	url = ../packages/Win32.git
 	ignore = untracked
 [submodule "libraries/primitive"]
 	path = libraries/primitive
-	url = http://git.haskell.org/packages/primitive.git
+	url = ../packages/primitive.git
 	ignore = untracked
 [submodule "libraries/vector"]
 	path = libraries/vector
-	url = http://git.haskell.org/packages/vector.git
+	url = ../packages/vector.git
 	ignore = untracked
 [submodule "libraries/time"]
 	path = libraries/time
-	url = http://git.haskell.org/packages/time.git
+	url = ../packages/time.git
 	ignore = untracked
 [submodule "libraries/random"]
 	path = libraries/random
-	url = http://git.haskell.org/packages/random.git
+	url = ../packages/random.git
 	ignore = untracked