[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?

Comienza en src/configLoader/loader.js, en las líneas 30-36, donde se encuentra la comprobación comentada de la configuración de package.json. Traza cómo se descubren los archivos de configuración desde un subdirectorio hacia la raíz del repositorio y verifica el comportamiento con y sin una clave config.commitizen. Se considera terminado cuando package.json solo se acepta si declara esa clave, sin romper otras fuentes de configuración.

Escrito por el modelo de indexación a partir del texto del issue.