Fix dune-project dependency calculation to scan all opam files in subtrees
Previously, collect_external_deps only looked at dependencies from packages
registered in the opam overlay. This missed dependencies from .opam files
that exist in monorepo subtree directories but aren't in the overlay.
Now scans each subtree directory for all .opam files and takes the union
of their dependencies for the root dune-project.
Also adds a warning to `monopam status` when local opam files aren't
registered in the overlay, helping identify packages that should be added.
Fixes String.Map usage in verse.ml (changed to Hashtbl).
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>