__before_bundle directory should be inside the engine folder.

modules/dts-bundler/src/index.ts

@@ -48,7 +48,7 @@ export async function build (options: Options): Promise<boolean> {
 
     const tsConfigPath = statsQuery.tsConfigPath;
 
-    const unbundledOutDir = ps.join(outDir, '__before_bundle');
+    const unbundledOutDir = ps.join(engine, '__dts_before_bundle');
     const parsedCommandLine = ts.getParsedCommandLineOfConfigFile(
         tsConfigPath, {
             declaration: true,

modules/utils/src/path.ts

@@ -11,6 +11,10 @@ export function dirname (path: string): string {
     return formatPath(ps.dirname(path));
 }
 
+export function basename (path: string): string {
+    return formatPath(ps.basename(path));
+}
+
 export function join (...args: string[]): string {
     return formatPath(ps.join(...args));
 }

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@cocos/ccbuild",
-  "version": "2.2.4",
+  "version": "2.2.5",
   "description": "The next generation of build tool for Cocos engine.",
   "main": "./lib/index.js",
   "types": "./lib/index.d.ts",