pcp::find_layer fails to resolve relative reference paths

[bresilla]

pcp::index::find_layer only matches asset_path against canonical absolute layer identifiers via exact / suffix comparison, so a needle like @../Materials/Materials.usd@ never matches the canonical absolute identifier already present in the layer stack. Any sub-USD that references a sibling via parent traversal therefore raises UnresolvedLayer and its sub-graph never enters the composed stage — surfaces e.g. as missing Looks/ namespaces on multi-file asset hierarchies. PR with fix + repro to follow.