Introduce demand-based-improvers (DBI) to purldb

When I fetch the data about a specific package from purldb, the details available may not always be of the highest quality, depending on the functionality of the tools and processes that initially created that purldb entry. Perhaps such a query should trigger a "quality check" that looks for values in the most interesting fields: license_expression, (valid) download_url, copyright, notice_text, etc. If the "quality check" fails, that should trigger a new demand-based-improver (DBI) process to perform all the necessary processes to find those interesting values, in a manner similar to creating a brand new entry. If possible, some status code or message should be returned that indicates that the DBI was initiated.