[Config loader] check if config key is in package.json before accepting it as a config file

Hello,

I'm working on a monorepo with backend and frontend and a `.czrc` at the directory's root to enforce commit rules with commitizen.

All frontend-related files are in a subdirectory, including the `package.json` file at different filesystem levels.

**My issue**: In a subdirectory with a package.json between `pwd` and git repository root, `commitizen` seems to acknowledge this file as a configuration file despite the lack of `config.commitizen` key.

**Fix proposal**: There seems to be a mechanism to check the package.json has a commitizen configuration, but it is commented: https://github.com/commitizen/cz-cli/blob/2e57fd02a5fd2b2dcf89d3235a6956edaeaf672c/src/configLoader/loader.js#L30-L36

I tried to uncomment it in a local version of commitizen, and it works as expected.
Would it be possible to uncomment it or does it have unwanted side effects?

Inizia in src/configLoader/loader.js alle righe 30-36, dove si trova il controllo commentato della configurazione di package.json. Segui come vengono individuati i file di configurazione da una sottodirectory verso la radice del repository e verifica il comportamento con e senza una chiave config.commitizen. Il lavoro è completato quando package.json viene accettato solo se dichiara quella chiave, senza compromettere altre fonti di configurazione.

Scritto dal modello di indicizzazione a partire dal testo della issue.