cmd/protoc-gen-go: don't chain public imports

Consider the case:

	a.proto publicly imports b.proto
	b.proto publicly imports c.proto

Should a.pb.go include symbols defined in c.pb.go?

Historically, it has not. As of #155677, it does. Regardless of which behavior
is preferable, #155677 produces broken code in some common situations: If
a.proto also publicly imports c.proto, we now generate two copies of the
forwarding decls for that file.

Restore the pre-#155677 behavior to avoid this breakage.

Change-Id: I283600b3be19eac2c3b3c14233bb69fa64661581
Reviewed-on: https://go-review.googlesource.com/c/156348
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

cmd/protoc-gen-go/internal_gengo/main.go

@@ -200,7 +200,7 @@ func genImport(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo, imp
 		gen.Error(err)
 		return
 	}
-	genForward := func(tok token.Token, name string) {
+	genForward := func(tok token.Token, name string, expr ast.Expr) {
 		// Don't import unexported symbols.
 		r, _ := utf8.DecodeRuneInString(name)
 		if !unicode.IsUpper(r) {
@@ -210,6 +210,13 @@ func genImport(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo, imp
 		if name == impFile.GoDescriptorIdent.GoName {
 			return
 		}
+		// Don't import decls referencing a symbol defined in another package.
+		// i.e., don't import decls which are themselves public imports:
+		//
+		//	type T = somepackage.T
+		if _, ok := expr.(*ast.SelectorExpr); ok {
+			return
+		}
 		g.P(tok, " ", name, " = ", impFile.GoImportPath.Ident(name))
 	}
 	g.P("// Symbols defined in public import of ", imp.Path())
@@ -220,10 +227,14 @@ func genImport(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo, imp
 			for _, spec := range decl.Specs {
 				switch spec := spec.(type) {
 				case *ast.TypeSpec:
-					genForward(decl.Tok, spec.Name.Name)
+					genForward(decl.Tok, spec.Name.Name, spec.Type)
 				case *ast.ValueSpec:
-					for _, name := range spec.Names {
-						genForward(decl.Tok, name.Name)
+					for i, name := range spec.Names {
+						var expr ast.Expr
+						if i < len(spec.Values) {
+							expr = spec.Values[i]
+						}
+						genForward(decl.Tok, name.Name, expr)
 					}
 				case *ast.ImportSpec:
 				default:

cmd/protoc-gen-go/testdata/import_public/a.pb.go

@@ -20,7 +20,6 @@ const _ = proto.ProtoPackageIsVersion3 // please upgrade the proto package
 
 // Symbols defined in public import of import_public/sub/a.proto
 
-type Sub2Message = sub.Sub2Message
 type E = sub.E
 
 const E_ZERO = sub.E_ZERO