feat(monopam): add export and import commands
- Add `export` command (replaces `split`) for exporting opam files
from any project directory to opam-repo
- Add `import` command for adding git repos as subtrees
- Add mono_lock.ml for tracking imported packages
Format: `<name> <url>#<ref>` (opam-style URLs)
- Rename monorepo_pkg.ml to pkg.ml
- Remove deprecated cmd_split.ml