Delete test.hs as it hardly tests anything
Rename Tool to Compiler, change naming scheme

The Compiler module is now designed to be imported qualified
Compiler: rename extensions to fileExtensions
Remove unused imports
Re-export Cabal integration from D.HS.Compiler and hide D.HS.Cabal
Change interface of runModuleT and evalModuleT

Both runModuleT and evalModuleT now automatically search for info files.

Also, evalModuleT does not accept the initial module info mapping — instead,
the empty mapping is used.
Document module monad stuff
Move readPackagesInfo to PackageDB
Rename Helpers to Modules
Rename PackageDB module to Packages
Docs
Don't re-export findModuleFile as it shouldn't be needed anymore
Rename defaultMain to main