fix(go_indexer): detect top-level when using file-level scopes (#5817)

kythe/go/indexer/BUILD

@@ -69,6 +69,7 @@ go_indexer_test(
     has_marked_source = True,
     import_path = "builtin",
     resolve_code_facts = True,
+    use_file_as_top_level_scope = True,
 )
 
 go_indexer_test(

kythe/go/indexer/emit.go

@@ -1224,7 +1224,7 @@ func (e *emitter) writeBinding(id *ast.Ident, kind string, parent *spb.VName) *s
 		return nil
 	}
 	target := e.pi.ObjectVName(obj)
-	if e.pi.ImportPath == "builtin" && parent != nil && parent.GetSignature() == "package" {
+	if e.pi.ImportPath == "builtin" && parent != nil && (parent.GetSignature() == "package" || parent.GetSignature() == "") {
 		// Special-case top-level builtin bindings: https://pkg.go.dev/builtin
 		target = govname.Builtin(id.String())
 		kind = "tbuiltin"

kythe/go/indexer/testdata/builtin.go

@@ -1,9 +1,12 @@
 package builtin
 
+//- _? defines/binding BoolBuiltin
 //- BoolBuiltin=vname("bool#builtin", "golang.org", "", "", "go").node/kind tbuiltin
 //- BoolBuiltin.code/rendered/signature "bool"
+//- _? defines/binding IntBuiltin
 //- IntBuiltin=vname("int#builtin", "golang.org", "", "", "go").node/kind tbuiltin
 //- IntBuiltin.code/rendered/signature "int"
 
+//- _? defines/binding ErrorBuiltin
 //- ErrorBuiltin=vname("error#builtin", "golang.org", "", "", "go").node/kind tbuiltin
 //- ErrorBuiltin.code/rendered/signature "error"