Return an error when adding the same asset label multiple times.

[andriyDev]

Objective

• Fixes #19026.

Solution

• Make the labeled assets in LoadContext be a &Mutex<HashMap>.
• Make LoadContexts for subassets reuse the same &Mutex<HashMap>.
• Immediately-loaded nested assets will naturally get a separate Mutex<HashMap>, so their subassets will be managed separately.

Testing

• Added tests to show the various cases working.

[andriyDev]

@alice-i-cecile Not really. The problem is that we need to be able to differentiate between immediate nested asset loads and their subassets, from "root asset" subassets. This was the thing I missed in #15481, and not something that's easy to fix without gross hacks. This solution has the advantage that as soon as you add a duplicate subasset, you'll get the feedback.

I don't think we need to worry much about the Mutex. It's got quite a small critical section, and likely has very low contention (people are probably not adding many subassets from many threads), plus asset loading in general is IO-bound.