[nix-local-build] Post-merge tasks

[23Skidoo]

We decided to postpone 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).

[23Skidoo]

Related: #3169.

[dcoutts]

Glob improvements & tests nearly ready. Will make a PR.