Introduce a data attribute to nixpkgs_package

[guibou]

This is a solution for #17, the new data attribute references all the files
needed to execute the nix expression, hence invalidating cache if they are
modified.

[mboes]

The extra sources files are not really data. They're extra source files. While #17 seems to claim a more general problem, AFAICS this is just a classic problem of a) not declaring all the inputs properly, b) not being able to. If we make nix_file a list attribute instead of a label attribute, then we can provide as many *.nix source files as we want. If Nixpkgs itself changes, then that will be captured by a change in the definition of nixpkgs_repository.

So in short, I think this is the right thing to do. But a spec by example should be closer to:

nixpkgs_package(
  name = "foo",
  srcs = glob(["*.nix"]),
  attribute_path = "foo",
  repository = "@nixpkgs",
)

[guibou]

@mboes the point of using data was that the "root" nix file was specified in nix_file and all the dependencies are in data. I can rename it srcs if you wish.

But what about nix_file, should we keep it or delete it? If we remove it, which file in srcs should be used? The order is not reliable because user may use glob to fill srcs. Should we use srcs AND nix_file?

[Fuuzetsu]

@mboes I don't think the issue is only up to nix files and data was a better name. The nix file itself can refer to arbitrary files: if the arbitrary file changes and the nix file doesn't, currently bazel will not re-run the role. I thought this was the original problem. Of course these arbitrary files could be other nix files but that's only a subset of possible inputs.

Complains addressed

[guibou]

@mboes name changed from data to srcs. And one of repository or path attribute should be setup.

[mboes]

@Fuuzetsu data is typically used for runtime dependencies. But there is no "runtime" here, and these are dependencies of the package metadata description. Technically the build_file could also have dependencies, since it can load(). So opting for renaming to nix_file_deps.