cabal new-build doesn't pass .proto files between deps in the same project

[judah]

Suppose package A depends on package B, both have codegen of proto files, and A's proto files depend on B's. We currently don't require A to also specify B's .proto files explicitly in extra-source-files. Instead, when proto-lens-setup installs B, it copies B's .proto files into the final "data" location, under the proto-lens-imports subdirectory. Then, when proto-lens-setup installs A, it looks for those files and passes the subdirectory when it calls protoc.

Unfortunately, this only works with stack or cabal install V1, and not cabal new-build when both packages are in the same cabal.project. new-build doesn't call the "copy" phase that we're using:
haskell/cabal#5933
Instead, it points at the source file data-dirs.

Ideally, we should figure out an alternative way for two packages to pass each other .proto files. It's annoying though that we have three ways to cover: stack, cabal V1 and cabal V2.