misc: fix serverComponentsExternalPackages usage for experimental.bundlePagesExternals

[feedthejim]

This PR fixes a bug where Next.js would not use the serverComponentsExternalPackages config when opting into the experimental.bundlePagesExternals flag.

The fix consists of just adding a check for the externals config.

[ijjk]

Tests Passed

[ijjk]

Stats from current PR

Default Build

General

	vercel/next.js canary	vercel/next.js feedthejim/fix-server-externals-pages-bundling	Change
buildDuration	11s	10.6s	N/A
buildDurationCached	5.8s	6.1s	⚠️ +235ms
nodeModulesSize	175 MB	175 MB	N/A
nextStartRea..uration (ms)	399ms	398ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js feedthejim/fix-server-externals-pages-bundling	Change
199-HASH.js gzip	30.5 kB	30.5 kB	N/A
3f784ff6-HASH.js gzip	53.2 kB	53.2 kB	✓
494.HASH.js gzip	182 B	182 B	✓
framework-HASH.js gzip	45.5 kB	45.5 kB	✓
main-app-HASH.js gzip	254 B	251 B	N/A
main-HASH.js gzip	33.1 kB	33.1 kB	N/A
webpack-HASH.js gzip	1.75 kB	1.75 kB	N/A
Overall change	98.9 kB	98.9 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js feedthejim/fix-server-externals-pages-bundling	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js feedthejim/fix-server-externals-pages-bundling	Change
_app-HASH.js gzip	205 B	205 B	✓
_error-HASH.js gzip	182 B	181 B	N/A
amp-HASH.js gzip	505 B	507 B	N/A
css-HASH.js gzip	322 B	323 B	N/A
dynamic-HASH.js gzip	2.59 kB	2.59 kB	N/A
edge-ssr-HASH.js gzip	258 B	259 B	N/A
head-HASH.js gzip	348 B	347 B	N/A
hooks-HASH.js gzip	369 B	368 B	N/A
image-HASH.js gzip	4.38 kB	4.38 kB	N/A
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	2.71 kB	2.7 kB	N/A
routerDirect..HASH.js gzip	318 B	318 B	✓
script-HASH.js gzip	384 B	383 B	N/A
withRouter-HASH.js gzip	319 B	320 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	885 B	885 B	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js feedthejim/fix-server-externals-pages-bundling	Change
_buildManifest.js gzip	483 B	484 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js feedthejim/fix-server-externals-pages-bundling	Change
index.html gzip	529 B	527 B	N/A
link.html gzip	541 B	540 B	N/A
withRouter.html gzip	523 B	522 B	N/A
Overall change	0 B	0 B	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js feedthejim/fix-server-externals-pages-bundling	Change
edge-ssr.js gzip	96.4 kB	96.4 kB	N/A
page.js gzip	141 kB	141 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js feedthejim/fix-server-externals-pages-bundling	Change
middleware-b..fest.js gzip	626 B	626 B	✓
middleware-r..fest.js gzip	148 B	151 B	N/A
middleware.js gzip	24.8 kB	24.8 kB	N/A
edge-runtime..pack.js gzip	1.92 kB	1.92 kB	✓
Overall change	2.55 kB	2.55 kB	✓

Commit: 45dc4e3