`cabal watch` command

This ticket is intended to capture ideas for a `cabal watch` command to give functionality along the lines of `ghcid`. In particular, one should be able to "watch" a component just as one `new-repl`s a component. I would imagine that there would be a few knobs to turn on what happens when something is altered. One could recompile with `no-code` (related issue: https://github.com/haskell/cabal/issues/1176) or perhaps send some form of signal to a running `ghci` process that just causes it to execute a `:r` automatically. In which case `cabal watch` would really be an option on `new-repl`. It might also be configurable to run a custom command. The main idea is that since cabal knows which files are in the dependency graph, it is a good place to stick a watch command on them.