[build-utils][fs-detectors][cli] MAJOR: move some of build-utils into new fs-detectors (#8054)

The `@vercel/build-utils` package was meant be shared functions necessary for writing a Vercel Builder (aka Runtime).

This package has since bloated into the catch-all package for anything that wasn't a Builder.

This PR removes the bloat in favor of a new package, `@vercel/fs-detectors`. It also removes the need for `@vercel/build-utils` to have a dependency on `@vercel/frameworks`.

- Related to #7951

packages/fs-detectors/test/integration.test.ts

@@ -0,0 +1,166 @@
+import path from 'path';
+import { promises } from 'fs';
+import { glob } from '@vercel/build-utils';
+import { detectBuilders } from '../src';
+const fs = promises;
+
+import {
+  packAndDeploy,
+  testDeployment,
+  // @ts-ignore
+} from '../../../test/lib/deployment/test-deployment';
+
+jest.setTimeout(4 * 60 * 1000);
+
+const builderUrl = '@canary';
+let buildUtilsUrl: string;
+
+beforeAll(async () => {
+  const buildUtilsPath = path.resolve(__dirname, '..');
+  buildUtilsUrl = await packAndDeploy(buildUtilsPath);
+  console.log('buildUtilsUrl', buildUtilsUrl);
+});
+
+it('Test `detectBuilders` and `detectRoutes`', async () => {
+  const fixture = path.join(__dirname, 'fixtures', '01-zero-config-api');
+  const json = await fs.readFile(path.join(fixture, 'package.json'), 'utf8');
+  const pkg = JSON.parse(json);
+  const fileList = await glob('**', { cwd: fixture });
+  const files = Object.keys(fileList);
+
+  const probes = [
+    {
+      path: '/api/my-endpoint',
+      mustContain: 'my-endpoint',
+      status: 200,
+    },
+    {
+      path: '/api/other-endpoint',
+      mustContain: 'other-endpoint',
+      status: 200,
+    },
+    {
+      path: '/api/team/zeit',
+      mustContain: 'team/zeit',
+      status: 200,
+    },
+    {
+      path: '/api/user/myself',
+      mustContain: 'user/myself',
+      status: 200,
+    },
+    {
+      path: '/api/not-okay/',
+      status: 404,
+    },
+    {
+      path: '/api',
+      status: 404,
+    },
+    {
+      path: '/api/',
+      status: 404,
+    },
+    {
+      path: '/',
+      mustContain: 'hello from index.txt',
+    },
+  ];
+
+  const { builders, defaultRoutes } = await detectBuilders(files, pkg);
+
+  const nowConfig = { builds: builders, routes: defaultRoutes, probes };
+
+  await fs.writeFile(
+    path.join(fixture, 'now.json'),
+    JSON.stringify(nowConfig, null, 2)
+  );
+
+  const deployment = await testDeployment(
+    { builderUrl, buildUtilsUrl },
+    fixture
+  );
+  expect(deployment).toBeDefined();
+});
+
+it('Test `detectBuilders` with `index` files', async () => {
+  const fixture = path.join(__dirname, 'fixtures', '02-zero-config-api');
+  const json = await fs.readFile(path.join(fixture, 'package.json'), 'utf8');
+  const pkg = JSON.parse(json);
+  const fileList = await glob('**', fixture);
+  const files = Object.keys(fileList);
+
+  const probes = [
+    {
+      path: '/api/not-okay',
+      status: 404,
+    },
+    {
+      path: '/api',
+      mustContain: 'hello from api/index.js',
+      status: 200,
+    },
+    {
+      path: '/api/',
+      mustContain: 'hello from api/index.js',
+      status: 200,
+    },
+    {
+      path: '/api/index',
+      mustContain: 'hello from api/index.js',
+      status: 200,
+    },
+    {
+      path: '/api/index.js',
+      mustContain: 'hello from api/index.js',
+      status: 200,
+    },
+    {
+      path: '/api/date.js',
+      mustContain: 'hello from api/date.js',
+      status: 200,
+    },
+    {
+      // Someone might expect this to be `date.js`,
+      // but I doubt that there is any case were both
+      // `date/index.js` and `date.js` exists,
+      // so it is not special cased
+      path: '/api/date',
+      mustContain: 'hello from api/date/index.js',
+      status: 200,
+    },
+    {
+      path: '/api/date/',
+      mustContain: 'hello from api/date/index.js',
+      status: 200,
+    },
+    {
+      path: '/api/date/index',
+      mustContain: 'hello from api/date/index.js',
+      status: 200,
+    },
+    {
+      path: '/api/date/index.js',
+      mustContain: 'hello from api/date/index.js',
+      status: 200,
+    },
+    {
+      path: '/',
+      mustContain: 'hello from index.txt',
+    },
+  ];
+
+  const { builders, defaultRoutes } = await detectBuilders(files, pkg);
+
+  const nowConfig = { builds: builders, routes: defaultRoutes, probes };
+  await fs.writeFile(
+    path.join(fixture, 'now.json'),
+    JSON.stringify(nowConfig, null, 2)
+  );
+
+  const deployment = await testDeployment(
+    { builderUrl, buildUtilsUrl },
+    fixture
+  );
+  expect(deployment).toBeDefined();
+});