Remove labeled_assets from LoadedAsset and ErasedLoadedAsset

[andriyDev]

Objective

Fixes #15417.

Solution

• Remove the labeled_assets fields from LoadedAsset and ErasedLoadedAsset.
• Created new structs CompleteLoadedAsset and CompleteErasedLoadedAsset to hold the labeled_subassets.
• When a subasset is LoadContext::finished, it produces a CompleteLoadedAsset.
• When a CompleteLoadedAsset is added to a LoadContext (as a subasset), their labeled_assets are merged, reporting any overlaps.

One important detail to note: nested subassets with overlapping names could in theory have been used in the past for the purposes of asset preprocessing. Even though there was no way to access these "shadowed" nested subassets, asset preprocessing does get access to these nested subassets. This does not seem like a case we should support though. It is confusing at best.

Testing

• This is just a refactor.

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Migration Guide

• Most uses of LoadedAsset and ErasedLoadedAsset should be replaced with CompleteLoadedAsset and CompleteErasedLoadedAsset respectively.

[andriyDev]

Just to make it clear what the rationale for this is. This makes implementing asset saving easier, since we don't need to deal with recursive types. We just have to borrow the root assets, and borrow the subassets and we're done. Without this PR, for each subasset, we'd need to create a new type like SubAssetHolder that holds the borrow to the subasset, and a Vec of SubAssetHolder which in turn hold borrows to the nested subassets (and so on and so on).

[andriyDev]

Rebased after #15509. Nothing really affected.

[andriyDev]

Rebased since this PR was a little stale. I folded in the accidental doc comment fix into the first commit.

[andriyDev]

Rebase after #15487 was merged, fairly trivial overall. This also had the funny side-effect that I could remove an expect lint for "large error result type", since apparently removing the meta field was enough to consider the struct "small".