[Moddability] Add blueprint loading from subfolders in mods

[Noelle Lavenza]

Allow modders to separate their own ObjectBlueprints.xml files into an ObjectBlueprints subdirectory, like how vanilla game does.
A mod is attached with an example implementation, implemented using Harmony (and also includes a reimplementation of the vanilla loading functionality that doesn't rely on hardcoded filenames).

[Armithaig]

Yes this is planned/coming, we’re fencing opinions on our preferred way to load all the XML/RPM files.