Implement a file.Resolver decorator that composites a parent resolver
with child resolvers created from extracted archive contents. Files from
child resolvers appear transparently alongside parent files.
Key behaviors:
- FilesByPath/FilesByGlob/FilesByMIMEType delegate to parent AND children
- FileContentsByLocation routes to the correct resolver via FileSystemID
- Locations from children get archive-prefixed AccessPath (using ":"
separator matching Java cataloger convention) and unique FileSystemID
- Thread-safe via RWMutex for concurrent access
Signed-off-by: Thomas Bechtold <thomasbechtold@jpberlin.de>