Fix main.go generation when executing on a windows machine

When normalizing the `importPath` it will try to write on `main.go` files with imports
like `github.com/caddyserver/xcaddy/c:\xcaddy`

cmd/xcaddy/main.go

@@ -199,7 +199,7 @@ func runDev(ctx context.Context, args []string) error {
 	if err != nil {
 		return fmt.Errorf("unable to determine current directory: %v", err)
 	}
-	importPath := path.Join(currentModule, strings.TrimPrefix(cwd, filepath.ToSlash(moduleDir)))
+	importPath := normalizeImportPath(currentModule, cwd, moduleDir)
 
 	// build caddy with this module plugged in
 	builder := xcaddy.Builder{
@@ -243,6 +243,10 @@ func runDev(ctx context.Context, args []string) error {
 	return cmd.Wait()
 }
 
+func normalizeImportPath(currentModule string, cwd string, moduleDir string) string {
+	return path.Join(currentModule, filepath.ToSlash(strings.TrimPrefix(cwd, moduleDir)))
+}
+
 func trapSignals(ctx context.Context, cancel context.CancelFunc) {
 	sig := make(chan os.Signal, 1)
 	signal.Notify(sig, os.Interrupt)

cmd/xcaddy/main_test.go

@@ -71,3 +71,34 @@ func TestSplitWith(t *testing.T) {
 		}
 	}
 }
+
+func Test_normalizeImportPath(t *testing.T) {
+	type args struct {
+		currentModule string
+		cwd           string
+		moduleDir     string
+	}
+	tests := []struct {
+		name string
+		args args
+		want string
+	}{
+		{"linux-path", args{
+			currentModule: "github.com/caddyserver/xcaddy",
+			cwd:           "/xcaddy",
+			moduleDir:     "/xcaddy",
+		}, "github.com/caddyserver/xcaddy"},
+		{"windows-path", args{
+			currentModule: "github.com/caddyserver/xcaddy",
+			cwd:           "c:\\xcaddy",
+			moduleDir:     "c:\\xcaddy",
+		}, "github.com/caddyserver/xcaddy"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := normalizeImportPath(tt.args.currentModule, tt.args.cwd, tt.args.moduleDir); got != tt.want {
+				t.Errorf("normalizeImportPath() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}