Unload handler for a dependency's loader.py code

[FichteFoll]

At this moment, dependencies that get uninstalled or updated especially will not be removed from sys.modules. To guarantee the proper latest version of a depency to be available form import in case of a package and dependency update, the module would have to be re-imported and the easiest way to do that would be to remove it from sys.modules (and all its submodules).

During that process, the (default) loader.py code could also undo its loading steps, such as adding the dependency to sys.path, and thus doing better at cleaning up.

I propose following a similar style to how ST unloads plugins: with a module attribute (global function).

Potential issues:

• Packages with the same name as the module, since the package path is on sys.path as well, but that is a problem when installing the dependency too.

[FichteFoll]

Since loader.py has inofficially been deprecated, will be removed in a future release, and dependency installation will be reworked in general, I believe this issue to be redundant. While still valid, surprisingly this didn't seem to have impacted anyone as far as I could tell, so it's most likely not worth fixing even for older versions.