Fix HMR when custom _app or _document is removed

[ijjk]

This adds the fallback webpack alias handling to handle a custom _app or _document being removed in development gracefully.

Bug

• Related issues linked using fixes #number
• Integration tests added
• Errors have helpful link attached, see contributing.md

Fixes: #27888

[ijjk]

Stats from current PR

Default Build (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
buildDuration	13.7s	13.4s	-237ms
buildDurationCached	3.4s	3.3s	-140ms
nodeModulesSize	61.5 MB	61.5 MB	⚠️ +3.07 kB

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.502	2.544	⚠️ +0.04
/ avg req/sec	999.05	982.65	⚠️ -16.4
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.616	1.641	⚠️ +0.02
/error-in-render avg req/sec	1546.97	1523.43	⚠️ -23.54

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
745.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	23.2 kB	23.2 kB	✓
webpack-HASH.js gzip	1.44 kB	1.44 kB	✓
Overall change	67.1 kB	67.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
_app-HASH.js gzip	979 B	979 B	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	329 B	329 B	✓
dynamic-HASH.js gzip	2.67 kB	2.67 kB	✓
head-HASH.js gzip	351 B	351 B	✓
hooks-HASH.js gzip	918 B	918 B	✓
image-HASH.js gzip	4.14 kB	4.14 kB	✓
index-HASH.js gzip	261 B	261 B	✓
link-HASH.js gzip	1.66 kB	1.66 kB	✓
routerDirect..HASH.js gzip	318 B	318 B	✓
script-HASH.js gzip	387 B	387 B	✓
withRouter-HASH.js gzip	320 B	320 B	✓
bb14e60e810b..30f.css gzip	125 B	125 B	✓
Overall change	13 kB	13 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
_buildManifest.js gzip	492 B	492 B	✓
Overall change	492 B	492 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
index.html gzip	539 B	539 B	✓
link.html gzip	551 B	551 B	✓
withRouter.html gzip	532 B	532 B	✓
Overall change	1.62 kB	1.62 kB	✓

---

Webpack 4 Mode (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
buildDuration	10.8s	10.9s	⚠️ +123ms
buildDurationCached	4.6s	4.5s	-155ms
nodeModulesSize	61.5 MB	61.5 MB	⚠️ +3.07 kB

Page Load Tests Overall increase ✓

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.57	2.557	-0.01
/ avg req/sec	972.93	977.75	+4.82
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.668	1.532	-0.14
/error-in-render avg req/sec	1498.56	1631.42	+132.86

Client Bundles (main, webpack, commons)

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
16.HASH.js gzip	186 B	186 B	✓
677f882d2ed8..HASH.js gzip	14.1 kB	14.1 kB	✓
framework.HASH.js gzip	41.9 kB	41.9 kB	✓
main-HASH.js gzip	10.6 kB	10.6 kB	✓
webpack-HASH.js gzip	1.19 kB	1.19 kB	✓
Overall change	68 kB	68 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
_app-HASH.js gzip	964 B	964 B	✓
_error-HASH.js gzip	3.8 kB	3.8 kB	✓
amp-HASH.js gzip	552 B	552 B	✓
css-HASH.js gzip	333 B	333 B	✓
dynamic-HASH.js gzip	2.87 kB	2.87 kB	✓
head-HASH.js gzip	3.06 kB	3.06 kB	✓
hooks-HASH.js gzip	924 B	924 B	✓
index-HASH.js gzip	231 B	231 B	✓
link-HASH.js gzip	1.64 kB	1.64 kB	✓
routerDirect..HASH.js gzip	298 B	298 B	✓
script-HASH.js gzip	2.98 kB	2.98 kB	✓
withRouter-HASH.js gzip	295 B	295 B	✓
30809af5c834..565.css gzip	125 B	125 B	✓
Overall change	18.1 kB	18.1 kB	✓

Client Build Manifests

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
_buildManifest.js gzip	500 B	500 B	✓
Overall change	500 B	500 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js fix/rm-app-hmr	Change
index.html gzip	585 B	585 B	✓
link.html gzip	597 B	597 B	✓
withRouter.html gzip	578 B	578 B	✓
Overall change	1.76 kB	1.76 kB	✓

Commit: 4c7c9b2