Split package.json into development specific content

[ilg-ul]

Split the project configuration (now all in package.json) and move parts specific to development into separate files.

For example:

  "xpacks": {
    "devIncludePackageFiles": [
      "package-xpm-develop.json",
      "tests/package-xpm.json"
    ]
  }

[ilg-ul]

To be noted that this requires a more elaborate logic to allow configuration updates when installing/uninstalling packages.