Uses/cabal.mk: Add support for building multi-repository cabal.projects.

To build such projects a port must add the repository to MASTER_SITES under
some group and then use this group in USE_CABAL.

Cabal2Tuple (ports-mgmt/hs-cabal2tuple) supports this feature starting with
version 2.0. Running make make-use-cabal will automatically pass necessary
flags to it.