Add --extra-pin flag and extra_repo_dirs to build_tool_from_repo
Repeatable --extra-pin DIR flag pins additional local checkouts.
Used for JTW's mime_printer dependency which lives in a separate repo.
Each extra dir gets its .opam files read and added to pins/source_dirs.
Tested: mime_printer pinned from ~/mime_printer, solve succeeds,
72/74 JTW tool packages build (js_top_worker needs 5.4.1 update).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>