Random build failure due to race condition in handle-externals

### Link to the code that reproduces this issue https://github.com/ashi009/nextjs-wasm ### To Reproduce https://github.com/vercel/next.js/blob/a71a764cae4e92fb3b150d157405245be2e99ddd/packages/next/src/build/handle-externals.ts#L327-L346 At line 327, the `resolvedExternalPackageDirs` is initialized at the first invocation of `async handleExternals()`. However, the content of `resolvedExternalPackageDirs` will not be populated until line 342. https://github.com/vercel/next.js/blob/a71a764cae4e92fb3b150d157405245be2e99ddd/packages/next/src/build/handle-externals.ts#L347-L356 Another invocation of `async handleExternals()` will skip initializing `resolvedExternalPackageDirs`, as it is already a `Map` instance. So, it will continue to line 347 and pass `resolvedExternalPackageDirs` to `resolveBundlingOptOutPackages`. **RACE**: Depending on the execution of the first invocation, the content in `resolvedExternalPackageDirs` may or may not be populated. The `resolvedBundlingOptOutRes` is undetermined. ### Current vs. Expected behavior current: external dependencies may not be resolved when calling `resolveBundlingOptOutPackages`. Which causes random build failures, like https://github.com/rustwasm/wasm-pack/issues/1402. expected: external dependencies must be resolved first. ### Provide environment information ```bash Operating System: Platform: darwin Arch: arm64 Version: Darwin Kernel Version 24.1.0: Thu Oct 10 21:03:11 PDT 2024; root:xnu-11215.41.3~2/RELEASE_ARM64_T6020 Available memory (MB): 32768 Available CPU cores: 12 Binaries: Node: 22.11.0 npm: 10.9.0 Yarn: 1.22.22 pnpm: N/A Relevant Packages: next: 15.1.6 // There is a newer version (15.1.7) available, upgrade recommended! eslint-config-next: 15.1.5 react: 18.3.1 react-dom: 18.3.1 typescript: 5.7.3 Next.js Config: output: N/A ``` ### Which area(s) are affected? (Select all that apply) Webpack ### Which stage(s) are affected? (Select all that apply) next build (local), next dev (l…