Having a machanism to rewrite res:// paths when mounting pck

[TML233]

To implement mounting pcks under a specific resource path, we need to find a way of rewriting res:// path references when mounting pck. This makes the res:// path references in the exported pck still being valid when they are mounted under a specific resource path in a running game.
Say you have a base game, and it mounts a mod_foo.pck file under its res://mods/foo/ folder. The file res://image.png of the modpack are now placed under res://mods/foo/image.png in the base game. So when a scene in the modpack accesses res://image.png, the access path needs to be rewritten to res://mods/foo/image.png to ensure the resource dependencies of the modpack still integrated. And the scripts in the modpack can also hard-code res:// paths in the code.