Handle modules with no data to migrate

Follow-up to #2569771: [meta] Highlighting of source modules with no upgrade paths

Right now, for each enabled module on the source site, if we have no migrations tied to importing data from the module, we simply display "No upgrade path available". We should do our best to identify the possible scenarios, giving more specific details for each one and applying "warning" or "error" highlighting as appropriate.

Module with no data (tables or variables) to migrate - should not even be shown. The problem here is there is no way to programmatically determine whether the source module has data to migrate. We can hard-code this knowledge for core modules, and perhaps the most popular contrib modules, but we can't reliably determine this in the general case, short of populating module metadata on d.o and retrieving it from there.