Remove nixpkgs_git_repository, it clashes with nix_file_deps heuristics

[Profpatsch]

A minimal WORKSPACE setup of rules_haskell that uses nixpkgs_git_repository fails on current master because nixpkgs_git_repository checks out nixpkgs into a bazel repository, which makes the heuristic require to manually specify every single file from the checked out nixpkgs. See this discission.

@guibou says:

You can imagine a even more minimal setup such as:

haskell_register_ghc_nixpkgs(
   version = "8.4.3",
   repositories = { "nixpkgs": ":nixpkgs.nix" }
)

Where nixpkgs.nix contains the relevant fetchTarball call. This solution does not use nixpkgs_git_repository and is more reproducible. Actually, I just realized that nixpkgs_git_repository does not forces the usage of the sha256 argument and hence is not reproducible.

So the best plan of action we figured out is to ask the user to use the nix-based fetchTarball approach and remove nixpkgs_git_repository.

cc @regnat @mboes

[samrose]

@Profpatsch is this still considered an issue for rules_nixpkgs ?

I took note that this was posted on 2019 (although I do see it is labelled "priority 1").

If someone were to submit a pull request on this: are you simply looking to remove nixpkgs_git_repository plus update the examples? It appears that nix_local_repository already covers the minimal setup example you have in the original issue.

[aherrmann]

@samrose Thanks for the ping! This has been fixed in the meantime, the repository now also includes working examples that use nixpkgs_git_repository.

[samrose]

@aherrmann thanks! If someone wants to contribute to this repo, what do you think are some good inroads toward PR if any?

[aherrmann]

@samrose this one may be a good starting point: #143