feat: get react-native package name from package.json in codegen script (#46604)

Summary:
This PR fixes the retrieval of react native package name to retrieve it from package.json.

This fixes the annoying message in codegen that poped up when installing pods:

```
[Codegen] Found react-native-macos
[Codegen] CodegenConfig Deprecated Setup for react-native-macos.
    The configuration file still contains the codegen in the libraries array.
    If possible, replace it with a single object.

BEFORE:
    {
      // ...
      "codegenConfig": {
        "libraries": [
          {
            "name": "libName1",
            "type": "all|components|modules",
            "jsSrcsRoot": "libName1/js"
          },
          {
            "name": "libName2",
            "type": "all|components|modules",
            "jsSrcsRoot": "libName2/src"
          }
        ]
      }
    }

    AFTER:
    {
      "codegenConfig": {
        "name": "libraries",
        "type": "all",
        "jsSrcsRoot": "."
      }
    }
```

## Changelog:

[GENERAL] [CHANGED] - get react-native package name from package.json in codegen script

Pull Request resolved: https://github.com/facebook/react-native/pull/46604

Test Plan: Install pods

Reviewed By: cortinico

Differential Revision: D63335543

Pulled By: arushikesarwani94

fbshipit-source-id: 0470e54f0fc7aa962918c889855c52648d11e32d