General Actions:
Reference core extensions. The extensions that are part of the sealed distribution and which are not upgradable.
Reference local and installed extensions.
Resolve extension by searching in all registered remote ExtensionRepository objects.
Take care of the extension type specific actions.
Create ExtensionRepository objects.
Is called to apply extension type specific installation/uninstallation/upgrade actions (load components from jar extensions, import xar pages, etc.).
Provide a list of ExtensionRepositoryId representing the remote repositories where ExtensionRepositoryManager will search.