cmd/go: list -json -cgo omits Imports in files generated by cgo

[alandonovan]

$ go version
go version devel +4f172e7a75 Mon Jun 25 18:17:49 2018 +0000 linux/amd64
$ ls ./cgo/
a.go
$ cat ./cgo/a.go 
package cgo

import "C"

var V = C.malloc(1)
$ go list -json ./cgo
{
        "Dir": ".../src/golang.org/x/tools/cgo",
        "ImportPath": "golang.org/x/tools/cgo",
        "Name": "cgo",
        "CgoFiles": [
                "a.go"
        ],
        "Imports": [
                "C"
        ],
        "Deps": [
                "internal/bytealg",
                "internal/cpu",
                "internal/race",
                "runtime",
                "runtime/cgo",
                "runtime/internal/atomic",
                "runtime/internal/sys",
                "sync",
                "sync/atomic",
                "syscall",
                "unsafe"
        ]
}
$ go list -json -cgo ./cgo
{
        "Dir": ".../src/golang.org/x/tools/cgo",
        "ImportPath": "golang.org/x/tools/cgo",
        "Name": "cgo",
        "CgoFiles": [
                "~/.cache/go-build/f3/f3242b8ad8bca1ad043e6df53b8b2887d860942c44c657b28f91bafead6f475b-d",
                "~/.cache/go-build/75/75d0efe60532158a29872e4b53737ce3eb1c5f3b0311bea3cca2a20d7f65b8d2-d",
                "~/.cache/go-build/55/55b3e499038558b20ae1f6fb3f86f1fbe98f77c6dd0a2afd2dc29036a25edbc9-d"
        ],
        "Imports": [
                "C"
        ],
        "Deps": [
                "internal/bytealg",
                "internal/cpu",
                "internal/race",
                "runtime",
                "runtime/cgo",
                "runtime/internal/atomic",
                "runtime/internal/sys",
                "sync",
                "sync/atomic",
                "syscall",
                "unsafe"
        ]
}

When the -cgo flag is enabled, go list processes Go source files that import "C" and lists the resulting generated files. However, the Imports list continues to contain "C" and does not reflect the actual imports of the generated files: unsafe, runtime/cgo, syscall.

[bcmills]

(CC: @rsc, but I'll try to get to this if he doesn't first.)

[gopherbot]

Change https://golang.org/cl/128935 mentions this issue: cmd/go: report implicit cgo inputs in go list -compiled