fix(monopam): use sources.toml for opam-repo URLs and fix tangled format

- opam_sync: delegate derive_urls to Pkg.derive_dev_repo so sources.toml
overrides are respected (fixes memtrace getting github URLs in opam-repo)
- dune_project: update tangled URL format from tangled.sh/@host/repo.git
to tangled.org/host/repo to match current API
- pkg: expose derive_dev_repo in .mli
- Add 35 new tests for derive_dev_repo priority and dune-project URL generation