stack_snapshot build executable components

[aherrmann]

Depends on #1303.

Adds a new attribute components to stack_snapshot which allows users to specify which Cabal components to build for a given package in the snapshot (defaults to building the library component). E.g. for the proto-lens-protoc package a user could specify the following to build both the library and executable components.

    components = {
        "proto-lens-protoc": [
            "lib",
            "exe",
        ],
    },

This reduces the need for dedicated http_archive definitions for executable components of Cabal packages.

The executable targets are available under labels of the form @<snapshot>-exe//<package_name>:<exe_name>, as discussed in #1306. For executable components that share the package's name the short form @<snapshot>-exe//<package> applies. For technical reasons (described in #1306) these executable targets are aliases pointing to haskell_cabal_binary targets of the form @<snapshot>//:_<package>_exe_<exe_name>. These labels are considered an implementation detail and not publicly visible.

The dependency graph generated by stack captures the library and executable dependencies of a package. When generating haskell_cabal_binary targets based on declared exe components, we pass in all package dependencies as well as the library component (if present).

The dependency graph generated by stack does not provide information on which components a package includes. Therefore, there is no easy way to automatically determine the set of components. However, only few packages define executable components, so it is feasible for users to manually define them as needed.

A set of known executable components is included in rules_haskell for common packages such as alex or happy.

proto-lens-protoc is converted from a dedicated http_archive to use the components attribute instead. This functions as a test-case for this feature.

ghcide is also converted from a dedicated http_archive to use the components attribute. The ghcide use-case documentation is updated accordingly.

Note, this PR does not yet inject executable components into the tools attribute of generated Cabal targets. E.g. alex and happy still have to be passed into stack_snapshot using the tools attribute. This will be addressed in a follow-up PR.

[guibou]

Why only "lib"?

[aherrmann]

That forms the default set of package components. I.e. by default we say a package only has a library component. (dict.pop(key, default=unbound))

[dpulls]

🎉 All dependencies have been resolved !

[dpulls]

💔 Pull request #1303's state different from open.

[Profpatsch]

broken_heart Pull request #1303's state different from open.

Hm, what does this message mean?

[aherrmann]

broken_heart Pull request #1303's state different from open.

Hm, what does this message mean?

I updated the PR message and didn't remove the "depends on" line. The PR that this one depends on has since been merged, so dpulls complains about depending on a closed PR. However, the status dpulls emits is green, so it can be ignored.