Make `go_bazel_test` pass with Bzlmod enabled

This is a quick fix for the test runner only, the tests themselves still rely on WORKSPACE (via local_repository).
The rules_go repo is also hardcoded, which means that other external repos likely won't work.

go/tools/bazel_testing/bazel_testing.go

diff --git a/go/tools/bazel_testing/bazel_testing.go b/go/tools/bazel_testing/bazel_testing.go
index a414b40..8a3fd02 100644
--- a/go/tools/bazel_testing/bazel_testing.go
+++ b/go/tools/bazel_testing/bazel_testing.go
@@ -101,6 +101,8 @@
 // instead of running tests.
 const debug = false
 
+var bzlmodEnabled = os.Getenv("BZLMOD_ENABLED") == "True"
+
 // outputUserRoot is set to the directory where Bazel should put its internal files.
 // Since Bazel 2.0.0, this needs to be set explicitly to avoid it defaulting to a
 // deeply nested directory within the test, which runs into Windows path length limits.
@@ -342,18 +344,18 @@
 		return "", cleanup, fmt.Errorf("building main workspace: %v", err)
 	}
 
+	var runfilesWorkspaceName string
 	// If some of the path arguments are missing an explicit workspace,
 	// read the workspace name from WORKSPACE. We need this to map arguments
 	// to runfiles in specific workspaces.
 	haveDefaultWorkspace := false
-	var defaultWorkspaceName string
 	for _, argPath := range files {
 		workspace, _, err := parseLocationArg(argPath)
 		if err == nil && workspace == "" {
 			haveDefaultWorkspace = true
 			cleanPath := path.Clean(argPath)
 			if cleanPath == "WORKSPACE" {
-				defaultWorkspaceName, err = loadWorkspaceName(cleanPath)
+				runfilesWorkspaceName, err = loadWorkspaceName(cleanPath)
 				if err != nil {
 					return "", cleanup, fmt.Errorf("could not load default workspace name: %v", err)
 				}
@@ -361,10 +363,18 @@
 			}
 		}
 	}
-	if haveDefaultWorkspace && defaultWorkspaceName == "" {
+	if haveDefaultWorkspace && runfilesWorkspaceName == "" {
 		return "", cleanup, fmt.Errorf("found files from default workspace, but not WORKSPACE")
 	}
 
+	var mainRepoName string
+	if bzlmodEnabled {
+		mainRepoName = runfilesWorkspaceName
+		runfilesWorkspaceName = "_main"
+	} else {
+		mainRepoName = runfilesWorkspaceName
+	}
+
 	// Index runfiles by workspace and short path. We need this to determine
 	// destination paths when we copy or link files.
 	runfiles, err := bazel.ListRunfiles()
@@ -379,20 +389,22 @@
 	}
 
 	// Copy or link file arguments from runfiles into fake workspace dirctories.
-	// Keep track of the workspace names we see, since we'll generate a WORKSPACE
+	// Keep track of the repo names we see, since we'll generate a WORKSPACE
 	// with local_repository rules later.
-	workspaceNames := make(map[string]bool)
+	canonicalRepoNames := make(map[string]bool)
 	for _, argPath := range files {
 		workspace, shortPath, err := parseLocationArg(argPath)
 		if err != nil {
 			return "", cleanup, err
 		}
+		key := runfileKey{workspace, shortPath}
 		if workspace == "" {
-			workspace = defaultWorkspaceName
+			workspace = mainRepoName
+			key.workspace = runfilesWorkspaceName
 		}
-		workspaceNames[workspace] = true
+		canonicalRepoNames[workspace] = true
 
-		srcPath, ok := runfileMap[runfileKey{workspace, shortPath}]
+		srcPath, ok := runfileMap[key]
 		if !ok {
 			return "", cleanup, fmt.Errorf("unknown runfile: %s", argPath)
 		}
@@ -422,10 +434,12 @@
 			NogoIncludes: args.NogoIncludes,
 			NogoExcludes: args.NogoExcludes,
 		}
-		for name := range workspaceNames {
-			info.WorkspaceNames = append(info.WorkspaceNames, name)
+		for name := range canonicalRepoNames {
+			info.WorkspaceNames = append(info.WorkspaceNames, makeApparent(name))
 		}
-		sort.Strings(info.WorkspaceNames)
+		sort.Slice(info.WorkspaceNames, func(i, j int) bool {
+			return info.WorkspaceNames[i].ApparentName < info.WorkspaceNames[j].ApparentName
+		})
 		if outBaseDir != "" {
 			goSDKPath := filepath.Join(outBaseDir, "external", "go_sdk")
 			rel, err := filepath.Rel(mainDir, goSDKPath)
@@ -474,6 +488,14 @@
 	return mainDir, cleanup, nil
 }
 
+func makeApparent(canonicalName string) workspaceName {
+	name := workspaceName{ApparentName: canonicalName, CanonicalName: canonicalName}
+	if canonicalName == "rules_go~"	|| canonicalName == "rules_go+" {
+		name.ApparentName = "io_bazel_rules_go"
+	}
+	return name
+}
+
 func extractTxtar(dir, txt string) error {
 	ar := txtar.Parse([]byte(txt))
 	for _, f := range ar.Files {
@@ -531,8 +553,13 @@
 	return name, nil
 }
 
+type workspaceName struct {
+	ApparentName  string
+	CanonicalName string
+}
+
 type workspaceTemplateInfo struct {
-	WorkspaceNames []string
+	WorkspaceNames []workspaceName
 	GoSDKPath      string
 	Nogo           string
 	NogoIncludes   []string
@@ -544,8 +571,8 @@
 var defaultWorkspaceTpl = template.Must(template.New("").Parse(`
 {{range .WorkspaceNames}}
 local_repository(
-    name = "{{.}}",
-    path = "../{{.}}",
+    name = "{{.ApparentName}}",
+    path = "../{{.CanonicalName}}",
 )
 {{end}}
 

go/tools/bazel_testing/def.bzl

diff --git a/go/tools/bazel_testing/def.bzl b/go/tools/bazel_testing/def.bzl
index d097027..ecc325e 100644
--- a/go/tools/bazel_testing/def.bzl
+++ b/go/tools/bazel_testing/def.bzl
@@ -43,6 +43,9 @@
                       ["-end_files"] +
                       kwargs["args"])
 
+    kwargs.setdefault("env", {})
+    kwargs["env"]["BZLMOD_ENABLED"] = str(str(Label("//:foo")).startswith("@@"))
+
     # Set rundir to the workspace root directory to ensure relative paths
     # are interpreted correctly.
     kwargs.setdefault("rundir", ".")