refactor: simplify swc loaders configuration

[huozhi]

Reduce the times that we re-create loaders, down to 3 loaders:

• swc loader for server layer
• swc loader for client layer
• babel loader

This change let us only create some loaders for once, and make the code easier to read.
babelLoader will only available when babel config is enabled, so we use filters to skip it when it's empty.

[ijjk]

Stats from current PR

Default Build

General

	vercel/next.js canary	vercel/next.js refactor/swc-loader	Change
buildDuration	10.7s	10.4s	N/A
buildDurationCached	5.9s	5.9s	N/A
nodeModulesSize	175 MB	175 MB	N/A
nextStartRea..uration (ms)	397ms	402ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js refactor/swc-loader	Change
199-HASH.js gzip	32.3 kB	32.4 kB	N/A
3f784ff6-HASH.js gzip	53.2 kB	53.2 kB	N/A
99.HASH.js gzip	182 B	182 B	✓
framework-HASH.js gzip	45.5 kB	45.5 kB	✓
main-app-HASH.js gzip	252 B	249 B	N/A
main-HASH.js gzip	35.3 kB	35.4 kB	N/A
webpack-HASH.js gzip	1.75 kB	1.75 kB	N/A
Overall change	45.7 kB	45.7 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js refactor/swc-loader	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js refactor/swc-loader	Change
_app-HASH.js gzip	206 B	205 B	N/A
_error-HASH.js gzip	182 B	180 B	N/A
amp-HASH.js gzip	506 B	505 B	N/A
css-HASH.js gzip	322 B	323 B	N/A
dynamic-HASH.js gzip	2.59 kB	2.59 kB	✓
edge-ssr-HASH.js gzip	260 B	259 B	N/A
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	369 B	369 B	✓
image-HASH.js gzip	4.38 kB	4.38 kB	N/A
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	2.67 kB	2.67 kB	N/A
routerDirect..HASH.js gzip	316 B	318 B	N/A
script-HASH.js gzip	385 B	384 B	N/A
withRouter-HASH.js gzip	319 B	319 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	3.99 kB	3.99 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js refactor/swc-loader	Change
_buildManifest.js gzip	484 B	482 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js refactor/swc-loader	Change
index.html gzip	528 B	529 B	N/A
link.html gzip	542 B	542 B	✓
withRouter.html gzip	524 B	524 B	✓
Overall change	1.07 kB	1.07 kB	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js refactor/swc-loader	Change
edge-ssr.js gzip	96 kB	96 kB	N/A
page.js gzip	140 kB	140 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js refactor/swc-loader	Change
middleware-b..fest.js gzip	626 B	621 B	N/A
middleware-r..fest.js gzip	150 B	151 B	N/A
middleware.js gzip	25.6 kB	25.6 kB	N/A
edge-runtime..pack.js gzip	1.92 kB	1.92 kB	✓
Overall change	1.92 kB	1.92 kB	✓

Commit: 9948998

[ijjk]

Tests Passed