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`.
If cabal.sandbox.config contains a "package-db:" declaration with the
wrong path and only the ghc version is wrong, for example:
```
package-db: <DIR>/.cabal-sandbox/x86_64-linux-ghc-7.8.3-packages.conf.d
```
Even though the user is using 7.10.0.20141222 `cabal repl` will correct
this and pass
```
-package-db <DIR>/.cabal-sandbox/x86_64-linux-ghc-7.10.0.20141222-packages.conf.d
```
to ghci, so obviously Cabal/cabal-install is doing some magic.
Conflicts:
Language/Haskell/GhcMod/PathsAndFiles.hs
Daniel Gröber