Convert loose sub-repos into proper submodules (re #8545)

Specifically, the following sub-repos/modules are converted:

 - libffi-tarballs
 - libraries/array
 - libraries/deepseq
 - libraries/directory
 - libraries/dph
 - libraries/filepath
 - libraries/haskell2010
 - libraries/haskell98
 - libraries/hoopl
 - libraries/hpc
 - libraries/old-locale
 - libraries/old-time
 - libraries/parallel
 - libraries/process
 - libraries/stm
 - libraries/unix
 - nofib
 - utils/hsc2hs

N.B. ghc-tarballs is not converted as it will probably be handled
     differently in the future.

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