cmd/go: list includes "C" in the Imports field but not in Deps

[mvdan]

$ go version
go version devel go1.21-f90b4cd655 Fri May 26 03:21:41 2023 +0000 linux/amd64
$ go list -f '{{.Imports}}{{"\n"}}{{.Deps}}' runtime/cgo
[C runtime/internal/sys sync sync/atomic unsafe]
[internal/abi internal/bytealg internal/coverage/rtcov internal/cpu internal/goarch internal/godebugs internal/goexperiment internal/goos internal/race runtime runtime/internal/atomic runtime/internal/math runtime/internal/sys runtime/internal/syscall sync sync/atomic unsafe]

Note how runtime/cgo includes C in its Imports field, but not in its Deps field. 1.20 is the same in this regard:

$ go version
go version go1.20.4 linux/amd64
$ go list -f '{{.Imports}}{{"\n"}}{{.Deps}}' runtime/cgo
[C runtime/internal/sys sync sync/atomic unsafe]
[internal/abi internal/bytealg internal/coverage/rtcov internal/cpu internal/goarch internal/goexperiment internal/goos internal/race runtime runtime/internal/atomic runtime/internal/math runtime/internal/sys runtime/internal/syscall sync sync/atomic unsafe]

The docs say:

        Imports      []string          // import paths used by this package
        Deps         []string          // all (recursively) imported dependencies

So I always naively assumed Imports to be a strict subset of Deps, since one is the set of direct imports, and the other is the set of all direct and indirect package dependencies (transitive imports). However, C breaks this rule.

I think Imports should stop including C, and here are my reasons why:

• Consistency; if Imports is not the list of direct dependencies, compared to the list of all transitive dependencies in Deps, then how do I get the list of direct dependencies? Right now I'm taking Imports and removing C from it, which feels hacky.
• C is not truly a Go package nor a useful import path; see how go list C fails in a funky way (which arguably could be fixed to give a better error, too).
• If a user or tool want to know whether a package uses CGo, they can already consult the CgoFiles field, which is clearer and more directly useful.
• Deps never included it either, so that's some proof that it's not really needed as a package dependency.

The only reason why one could argue that Imports should include C is that there really is an import "C" in the source code. However, Imports is not a direct representation of the import declarations in the source code; they are deduplicated, sorted, and omit any renames like import foo "bar" or import _ "baz".