Tracking ticket for Galois `new-build` requirements

I've been gathering assessments and feedback about the `new-build` features from my colleagues here at Galois. This ticket tracks the issues that we've found are particularly relevant for us, and that would need to be resolved before we can adopt `new-build` for most of our projects.

This is not meant to be exhaustive, or meant to imply that other issues are not important; this is just a scratchpad that'll evolve as I gather more feedback. Usernames in parentheses are the owners of this particular ticket from the Galois side, even when that's different from the assignee.
### Must resolve

These are the features that currently prevent us from adopting `new-build` on one or more projects:
- [x] #3417: Dependency executables aren't in PATH
- [x] #3586: Support `--prefix`, `--datadir`, etc
- [x] #3535: How do I haddock with new-build?
- [x] #3638: (@dmwit)
- [x] new-run
- [x] new-test
- [x] new-bench
- [x] new-exec
- [x] #4120: data files for inplace builds
### Nice to resolve

Not showstoppers, but necessary to make it worth switching from an existing system:
- [x] #2189: Support non-Hackage dependencies
- [x] #3332: new-install interface for installing executables (@dmwit)
- [x] nonlocal
- [x] local
- [ ] #3333: Garbage collecting the store (@dmwit)
- [x] #3391: Add flag to build all packages in project with new-build
- [x] #3394: new-build silent failure on zip-archive-0.3.0.2
- [ ] #3473: Replacement needed for `cabal copy`
- [ ] #3585: Add flag to `new-configure` for adding extra `packages` to `cabal.project.local` (@aisamanra)
- [ ] #3587: Allow specifying freeze file for other commands
### Bonus

Not necessarily specific `new-build`, but would be of great utility for adopters if we'll already be working in the neighborhood:
- [ ] #1493: build Setup.hs with local compiler when cross-compiling (@Ericson2314 laying the groundwork)