Make Classes Public For Extra Loaders Through Plugins

[coolGi69]

This PR just makes some classes public to allow for loader plugins to be created.
While both classes that are now made public aren't necessary to be to make a plugin, it makes it slightly easier.

[AlexIIL]

You should be able to make plugin icons via QuiltLoaderGui.createIcon(*)? (You're explicitly not meant to create String path icons yourself since they only read from loader's jar file).

Similarly, would you be okay if we added public api factory methods for ModDependencyImpl (in ModDependency) instead of making the classes public?

[coolGi69]

You should be able to make plugin icons via QuiltLoaderGui.createIcon(*)? (You're explicitly not meant to create String path icons yourself since they only read from loader's jar file).

Ah, alright. Missed that part when looking through the code when attempting to make the plugin.

Similarly, would you be okay if we added public api factory methods for ModDependencyImpl (in ModDependency) instead of making the classes public?

Yeah, sure. That's fine, I was just thinking to reduce code duplication this could be public.
I guess it's fine to implement it myself.