Move ConstraintSource to Distribution.Solver.* namespace

cabal-install/Distribution/Client/Config.hs

cabal-install/Distribution/Client/Configure.hs

cabal-install/Distribution/Client/Dependency.hs

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

cabal-install/Distribution/Client/Freeze.hs

cabal-install/Distribution/Client/Install.hs

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

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

cabal-install/Distribution/Client/ProjectPlanning.hs

...install/Distribution/Client/Sandbox/PackageEnvironment.hs

cabal-install/Distribution/Client/Setup.hs

cabal-install/Distribution/Client/Targets.hs

cabal-install/Distribution/Solver/Modular/Message.hs

cabal-install/Distribution/Solver/Modular/Preference.hs

cabal-install/Distribution/Solver/Modular/Tree.hs

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

cabal-install/cabal-install.cabal

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

...nstall/tests/UnitTests/Distribution/Solver/Modular/DSL.hs