Test plan for nix-local-build

[ezyang]

nix-local-build needs integration tests. Here are some of the tests I'd like to see, but please edit this ticket to add more.

• Single package without cabal.project, with dependencies on Hackage packages
• Simple cabal.project with multiple packages which depend on each other and depend on Hackage packages. If you build with --ghc-option, does this apply to local packages only and not Hackage packages?
• Vendor a package which exists on Hackage. The install plan should include both remote and local packages which depend on the vendored package. The vendored package should be tested with a version number that is the same as an existing Hackage upload, as well as a version number that is distinct from all Hackage uploads. The test should check that everything that depends on the vendored package is installed inplace only.

[ezyang]

CC @dcoutts, @23Skidoo

[dcoutts]

Started adding some in #3416

[jneira]

I wonder I those tests are still missing, the cabal test suite already contains quite integration tests