Cannot find module 'firebase-admin/app'

`package.json` requirements look like this –

```
"dependencies": {
"@sentry/node": "^5.30.0",
"body-parser": "1.19.0",
"express": "^4.17.1",
"firebase-admin": "10.0.0",
"mysql": "^2.18.1",
"pug": "3.0.2"
}
```

`node_modules` has the corresponding module in the path `~/node_modules/firebase-admin/lib/app`

Node code reads as –

```
const initializeApp = require('firebase-admin/app');

```

And when the app is run, the following stacktrace emerges.
```
Error: Cannot find module 'firebase-admin/app'
at Function.Module._resolveFilename (module.js:547:15)
at Function.Module._load (module.js:474:25)
at Module.require (module.js:596:17)
at require (internal/module.js:11:18)
at Object. (/var/www/html/project/backend/main.js:16:24)
at Module._compile (module.js:652:30)
at Object.Module._extensions..js (module.js:663:10)
at Module.load (module.js:565:32)
at tryModuleLoad (module.js:505:12)
at Function.Module._load (module.js:497:3)
```

Start with package.json and backend/main.js at line 16, then reproduce the reported require failure using firebase-admin 10.0.0. Inspect the installed firebase-admin/lib/app layout and the package's module entry points. Done means the Firebase Admin import resolves and the application starts without this error.

索引モデルが issue の本文から書いたものです。