Fix cabal install missing docs.json (followup)

Ok.  Been through everything wrt commit and discussion yesterday.
Think it's all done.

Biggest conceptual change is moving from using TemplateHaskell in LoadLibraries to loading docs.json at runtime.
This avoids using CPP, touch and other bits and replaces it with an "unsafeIO" to get the file loaded.

The other change is using process stuff in Setup.hs instead of the simpler system command.