This adds a way to specify the `evalSystem` or `evalPackages` explicitly when calling the `project` functions.
Currently if we want to make a `flake` that supports multiple systems we have few options:
* Require builders for all the supported systems (even just for `nix flake show`).
* Pass `--impure` so that haskell.nix can see `builtins.currentSystem` to set up `pkgs.evalPackages` to use that. Unfortunately this prevents nix from caching some of the work it does and often results in it recalculating for each supported system when it would otherwise be cached and take no time at all.
* Add an overlay to replace `evalPackages`. This works, but it is not straight forward.
* Materialize the nix files for the project.
This change allows `evalSystem = "x86_64-linux";` to be passed telling `haskell.nix` to run `cabal` and `nix-tools` on that system. The user will have to have a builder for that system, but does not need to have builders for the others (unless building outputs for them).
* Fix stack.yaml filtering when resolver file is in a subdirectory
`maybeCleanedSource` tries to include the resolver file into the cleaned
source, if a resolver is specified, but it was only working when the
file was in the root of the project, because all directories would be
filtered out.
* fixup! Fix stack.yaml filtering when resolver file is in a subdirectory
Fix calculating relPath for paths using subDir
* Add test case for local stack snapshot located in a subdirectory
Hamish Mackenzie
Shea Levy
Zhenya Vinogradov