fix: syncing of .lake/packages

[Vtec234]

• Add a mechanism for specifying which paths should be synced through collab-server vs excluded from sync. Excludes are not used here in the end but seem useful to keep.
• Mount package set packages as writable overlays. Sandbox reads go to the global cache normally, but sandbox writes are possible and persisted to the usual package directory. Files written in this way take precedence over package set files. Fixes Ensure users can persistently install packages in projects #24.