refactor: rename isAppDirEnabled to hasAppDir

[huozhi]

app dir is always "enabled", but not always exists. Rename to the proper word to describe. Found this confusion while developing other fetaures

Closes NEXT-2691

[ijjk]

Tests Passed

[ijjk]

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General

	vercel/next.js canary	vercel/next.js refactor/rename-args	Change
buildDuration	14s	13.9s	N/A
buildDurationCached	8.4s	6.4s	N/A
nodeModulesSize	198 MB	198 MB	N/A
nextStartRea..uration (ms)	436ms	440ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js refactor/rename-args	Change
2453-HASH.js gzip	30.2 kB	30.2 kB	N/A
3304.HASH.js gzip	181 B	181 B	✓
3f784ff6-HASH.js gzip	53.7 kB	53.7 kB	N/A
8299-HASH.js gzip	5.04 kB	5.04 kB	N/A
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	242 B	241 B	N/A
main-HASH.js gzip	32.1 kB	32.2 kB	N/A
webpack-HASH.js gzip	1.68 kB	1.68 kB	N/A
Overall change	45.4 kB	45.4 kB	✓

Legacy Client Bundles (polyfills)

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

Client Pages

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_app-HASH.js gzip	196 B	197 B	N/A
_error-HASH.js gzip	184 B	184 B	✓
amp-HASH.js gzip	505 B	505 B	✓
css-HASH.js gzip	324 B	325 B	N/A
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edge-ssr-HASH.js gzip	258 B	258 B	✓
head-HASH.js gzip	352 B	352 B	✓
hooks-HASH.js gzip	370 B	371 B	N/A
image-HASH.js gzip	4.2 kB	4.2 kB	✓
index-HASH.js gzip	259 B	259 B	✓
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routerDirect..HASH.js gzip	314 B	312 B	N/A
script-HASH.js gzip	386 B	386 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	6.56 kB	6.56 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js refactor/rename-args	Change
_buildManifest.js gzip	483 B	485 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js refactor/rename-args	Change
index.html gzip	530 B	530 B	✓
link.html gzip	541 B	542 B	N/A
withRouter.html gzip	524 B	524 B	✓
Overall change	1.05 kB	1.05 kB	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js refactor/rename-args	Change
edge-ssr.js gzip	95 kB	95 kB	N/A
page.js gzip	3.06 kB	3.07 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

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middleware-b..fest.js gzip	624 B	623 B	N/A
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middleware.js gzip	25.5 kB	25.5 kB	N/A
edge-runtime..pack.js gzip	839 B	839 B	✓
Overall change	990 B	990 B	✓

Next Runtimes

	vercel/next.js canary	vercel/next.js refactor/rename-args	Change
app-page-exp...dev.js gzip	171 kB	171 kB	✓
app-page-exp..prod.js gzip	96.7 kB	96.7 kB	✓
app-page-tur..prod.js gzip	98.5 kB	98.5 kB	✓
app-page-tur..prod.js gzip	92.9 kB	92.9 kB	✓
app-page.run...dev.js gzip	150 kB	150 kB	✓
app-page.run..prod.js gzip	91.4 kB	91.4 kB	✓
app-route-ex...dev.js gzip	21.3 kB	21.3 kB	✓
app-route-ex..prod.js gzip	15 kB	15 kB	✓
app-route-tu..prod.js gzip	15 kB	15 kB	✓
app-route-tu..prod.js gzip	14.8 kB	14.8 kB	✓
app-route.ru...dev.js gzip	20.9 kB	20.9 kB	✓
app-route.ru..prod.js gzip	14.7 kB	14.7 kB	✓
pages-api-tu..prod.js gzip	9.51 kB	9.51 kB	✓
pages-api.ru...dev.js gzip	9.79 kB	9.79 kB	✓
pages-api.ru..prod.js gzip	9.51 kB	9.51 kB	✓
pages-turbo...prod.js gzip	22.3 kB	22.3 kB	✓
pages.runtim...dev.js gzip	23 kB	23 kB	✓
pages.runtim..prod.js gzip	22.3 kB	22.3 kB	✓
server.runti..prod.js gzip	50.6 kB	50.6 kB	✓
Overall change	950 kB	950 kB	✓

build cache

	vercel/next.js canary	vercel/next.js refactor/rename-args	Change
0.pack gzip	1.56 MB	1.55 MB	N/A
index.pack gzip	106 kB	106 kB	N/A
Overall change	0 B	0 B	✓

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Commit: f01d674