Add makeRelativeTo{Cwd,Dir} file path utils
Add makeRelativeTo{Cwd,Dir} file path utils

Useful for reporting dir/file names in user interface code, e.g. in
notices and error messages.

Required by later nix-local-build branch patches.

gbaz/cabal,headprogrammingczar/cabal,gbaz/cabal,thomie/cabal,sopvop/cabal,thomie/cabal,Heather/cabal,themoritz/cabal,mydaum/cabal,lukexi/cabal,sopvop/cabal,Heather/cabal,mydaum/cabal,kolmodin/cabal,lukexi/cabal,themoritz/cabal,garetxe/cabal,themoritz/cabal,bennofs/cabal,headprogrammingczar/cabal,garetxe/cabal,sopvop/cabal,kolmodin/cabal,bennofs/cabal