Allow using `ProjectSettings.load_resource_pack()` into a subfolder

[nonchip]

Describe the project you are working on

multiple ones, isn't really project-specific but this would allow for anything from sandboxy keeping mods out of "core game files" to just simplifying a lot of tooling that works with packs.

Describe the problem or limitation you are having in your project

ProjectSettings.load_resource_pack can only "mount" to res://

Describe the feature / enhancement and how it helps to overcome the problem or limitation

add an optional parameter to ProjectSettings.load_resource_pack to allow mounting to any path in the virtual file system.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

essentially just need to track a single StringName across ProjectSettings.load_resource_pack, PackedData::add_pack, PackedSourcePCK::try_open_pack and prepend the String path built inside that latter function.

If this enhancement will not be used often, can it be worked around with a few lines of script?

no, none of the required functionality are exposed to GDScript nor GDExtension

Is there a reason why this should be core and not an add-on in the asset library?

because it would require a (non-compat-breaking) small api change to core functionality.

[nonchip]

note this is kinda related to #2689 and #389, but instead of implementing whole apis to interact with individual pack files and sandboxed scripting environments it would be a way simpler change that covers most use cases and honestly just "makes sense". every mount so far had a 2nd parameter ;)

[nonchip]

ok i appear too dumb to do the "autolink mention" right, so uh, my implementation attempt: godotengine/godot#82035

[Jade-TheCat]

Following up on some discussion in the PR and moving my comment at godotengine/godot#82035 (comment) here:

Perhaps instead of mangling res:// calls by prefixing them, we could have a separate location for those resources that acts like res:// for loaded packs?
So in the pack you would use pack:// for all pack-internal references and res:// for all of the base game resources. Then in project settings there could be a setting for where to load non-overriding packs by default (let's say it defaults to res://packs/<pack_file_name>), and there could be an alternative to load_resource_pack() (say, load_resource_pack_no_override() although that is quite long) that would load the pack to the path set in project settings. The engine would have to keep track of what pack a file is in, but I think that could be easy by checking if the file location relative to res:// was in the packs folder.
Of course, this would still allow for packs to be loaded into res://, say for updates or texture packs, but there would be the option to put the pack contents somewhere it doesn't mangle stuff for mods. I believe there's a way to specify what types of files get loaded by load_resource_pack() but if not that may be something to consider as well. That way you can specify per-pack whether it should override stuff and what it should override.

I'm not really sure how one would implement this as I have only looked at the engine code a little, but I might take a look into it when I have more time and if someone else doesn't pick up this or the linked PR doesn't get approved.

This is a larger proposed change than this original proposal, so if I need to move it to its own proposal I will, but since these are covering the same topic I figured putting it in the existing discussion would be better than having a bunch of issues opened

EDIT: I think this may also be useful for the people asking for GDScript sandboxing, as the engine keeping track of whether a script is in a pack could be used to automatically sandbox the script if that were an option to enable.