[nix-local-build] Post-merge tasks

We [decided to postpone](https://github.com/haskell/cabal/pull/3102#issuecomment-178220898) some tasks related to the `nix-local-build` code to get the branch merged quicker. The list of tasks in this ticket will be continuously updated.
- [x] Haddock comments for all top-level definitions
- [x] `D.C.RebuildMonad`
- [x] `D.C.FileMonitor` (mostly good already)
- [ ] Tests
- [x] `D.C.FileMonitor`
- [x] `D.C.Glob`
- [ ] `D.C.RebuildMonad` (may be unnecessary given that it's just a `FileMonitor` wrapper).
- [ ] #3322 integration tests for common scenarios / workflows
- [ ] #3430 test for windows MAX_PATH limitations, needs support for local tar packages to test installing into the store
- [ ] #3437 integration test for the 4th case for setup deps
- [ ] Project configuration
- [x] Decide how to support multiple builds with slightly different config without overwriting, e.g. different compilers or other settings.
- [ ] Explicit includes in `cabal.project` to clarify relationship between `~/.cabal/config`, `cabal.project`, `cabal.project.local`, `cabal.project.freeze` etc and appropriate comments in generated files (`.local` and `.freeze`).