Add ActiveRepos data type, add active-repositories config

Cabal/Distribution/Utils/GrammarRegex.hs

cabal-install/Distribution/Client/Config.hs

cabal-install/Distribution/Client/Get.hs

cabal-install/Distribution/Client/GlobalFlags.hs

cabal-install/Distribution/Client/IndexUtils.hs

cabal-install/Distribution/Client/IndexUtils/ActiveRepos.hs

cabal-install/Distribution/Client/Install.hs

cabal-install/Distribution/Client/ProjectConfig.hs

cabal-install/Distribution/Client/ProjectConfig/Legacy.hs

cabal-install/Distribution/Client/ProjectConfig/Types.hs

cabal-install/Distribution/Client/ProjectPlanning.hs

cabal-install/Distribution/Client/Setup.hs

cabal-install/Distribution/Solver/Types/PackageIndex.hs

cabal-install/cabal-install.cabal

cabal-install/cabal-install.cabal.pp

...tests/UnitTests/Distribution/Client/ArbitraryInstances.hs

...-install/tests/UnitTests/Distribution/Client/Described.hs

...tall/tests/UnitTests/Distribution/Client/ProjectConfig.hs

.../tests/UnitTests/Distribution/Client/TreeDiffInstances.hs