I have a plugin with a src folder structure following jest’s recommended practice of including a `__tests__` folder:
```
$ find src/
src/
src/document-html-props-server.ts
src/__tests__
src/__tests__/document-html-props-server.unit.ts
```
collect-plugins.ts [reads the src directory](960c18da53/packages/next/build/plugins/collect-plugins.ts (L65)) and finds:
```js
[ '__tests__', 'document-html-props-server.ts' ]
```
…which it then [converts to](960c18da53/packages/next/build/plugins/collect-plugins.ts (L76-L80)):
```js
[ '', 'document-html-props-server' ]
```
…and finally [outputs](960c18da53/packages/next/build/plugins/collect-plugins.ts (L90-L96)):
```
Next.js Plugin: {my plugin} listed invalid middleware
```
This pull request makes collect-plugins.ts only consider files, so my __tests__ directory and its contents are ignored.
07084d4819