Removed direct cabal imports in Linker and Finder modules

After reviewing the ghc and ghc-pkg dependencies wrt to Cabal modules, I noticed that PackageConfig and Packages, for the most part, are serving as a facade to Cabal modules. That is true except for Linker, Finder and GhcUtils, and this super small pull request rectifies that.

Here is a summary of the dependencies to Cabal modules (wrong dependencies, imho, are marked in red):

![ghc-2](https://f.cloud.github.com/assets/1887682/219436/f01fbe8a-852e-11e2-994b-f2f327b3370f.png)

With the exception of Packages.InstalledPackageId, most of the imports are minor and imo don't justify bring in Cabal as a whole for that. It seems to me that core dependencies such as InstalledPackageId should be either cloned (are there ghc specific fields here?) or moved to a pure library that could be shared by Cabal and GHC.