This turned out to be quite involved but save for this huge commit it's
actually quite awesome and squashes quite a few bugs and nasty
problems (hopefully). Most importantly we now have native cabal
component support without the user having to do anything to get it!
To do this we traverse imports starting from each component's
entrypoints (library modules or Main source file for executables) and
use this information to find which component's options each module will
build with. Under the assumption that these modules have to build with
every component they're used in we can now just pick one.
Quite a few internal assumptions have been invalidated by this
change. Most importantly the runGhcModT* family of cuntions now change
the current working directory to `cradleRootDir`.
Not doing this makes having GhcModT pretty pointless as users of the
library wouldn't be able to use custom inner monads as evey function for
dealing with GhcModT's would be constraint to (GhcModT IO) thus only
allowing IO as the inner monad.
- cradle now stores a list of active package databases instead of only
the user store (if present).
- rename `cradlePackageDb` -> `cradlePkgDbStack` as that`s what the ghc
documentaion calls this kind of thing
- `getPackageDbPackages` now returns names of all visible packages in
the given directory. Also the implementation now uses `ghc-pkg`
instead of manually looking at the package database
