In a separate goroutine, continuously fetch the oldest module in the
database from the module proxy and update it if necessary. Note that the
--crawl-interval flag must be specified to enable background fetching.
A few remaining questions:
- Should we crawl the imports of packages (to discover new packages not
in the database)? If we do, perhaps this should be opt-in as this
could cause the database to grow exponentially, which wouldn't be
desirable for small installations.