Use traverse instead of mapM all over the place

Cabal/Distribution/Compat/Binary/Class.hs

Cabal/Distribution/Compat/Graph.hs

Cabal/Distribution/PackageDescription/Check.hs

Cabal/Distribution/PackageDescription/Parse.hs

Cabal/Distribution/ParseUtils.hs

Cabal/Distribution/Simple.hs

Cabal/Distribution/Simple/Bench.hs

Cabal/Distribution/Simple/BuildTarget.hs

Cabal/Distribution/Simple/Compiler.hs

Cabal/Distribution/Simple/Configure.hs

Cabal/Distribution/Simple/GHC.hs

Cabal/Distribution/Simple/GHC/Internal.hs

Cabal/Distribution/Simple/GHCJS.hs

Cabal/Distribution/Simple/Haddock.hs

Cabal/Distribution/Simple/Hpc.hs

Cabal/Distribution/Simple/Install.hs

Cabal/Distribution/Simple/LHC.hs

Cabal/Distribution/Simple/LocalBuildInfo.hs

Cabal/Distribution/Simple/Program/Find.hs

Cabal/Distribution/Simple/Program/HcPkg.hs