Package dependency checking and handling in initialization and uninitialization

Implement the package dependency checking and handling in api_core_init() so that initializing and uninitializing of any package(s) can be performed at any given time correctly. There are already placeholders, with some comments, in the source file.

The actual implementation should be easy, but to make it clean it will require some of the other api functions, like api_core_checkversion and api_core_checkpackage, to be properly implemented as a side-effect. Some of those functions have not been fully designed yet.