cmd/go: vet: provide package ID to the vet tool

This field, which matches the IDs used by go list, will enable all vet
drivers to produce JSON output in a consistent format (a map from
package ID to analysis name to result).

Change-Id: Icac703b944de55df42c996dc2f672005014ad57a
Reviewed-on: https://go-review.googlesource.com/c/149960Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/work/exec.go

@@ -881,6 +881,7 @@ func (b *Builder) loadCachedSrcFiles(a *Action) bool {
 
 // vetConfig is the configuration passed to vet describing a single package.
 type vetConfig struct {
+	ID         string   // package ID (example: "fmt [fmt.test]")
 	Compiler   string   // compiler name (gc, gccgo)
 	Dir        string   // directory containing package
 	ImportPath string   // canonical import path ("package path")
@@ -914,6 +915,7 @@ func buildVetConfig(a *Action, srcfiles []string) {
 	// so that we can reformat them relative to the directory
 	// in which the go command is invoked.
 	vcfg := &vetConfig{
+		ID:          a.Package.ImportPath,
 		Compiler:    cfg.BuildToolchainName,
 		Dir:         a.Package.Dir,
 		GoFiles:     mkAbsFiles(a.Package.Dir, gofiles),