[meta] Highlighting of source modules with no upgrade paths

Follow-up to #2562849: [meta] Confirmation page

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. Possible scenarios:

1. 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. See #2569787: Handle modules with no data to migrate
2. Module exists in the D8 installation but is not enabled. To determine this we'll need a template discovery service that examines all modules, installed or not. Provide a message "This module must be enabled for the legacy content to be upgraded" or somesuch. Is this a warning or error? See #2568525: Identify potential (but unenabled) upgrade paths
3. Module exists in the D8 installation, is enabled, but does not provide an upgrade path (yet). What do we tell the upgrader in this case? "Please nag the module maintainer to provide an upgrade path"? Warning, perhaps, since there isn't much the upgrader can do about it. See #2569789: Handle modules which should have an upgrade path but don't
4. Module is not present in the D8 installation. Provide a message "To upgrade this module you must install and enable the Drupal 8 version of the module". Error? See #2569793: Handle modules which aren't present

In addition, perhaps we don't allow proceeding unless the upgrader has clicked a checkbox next to each no-upgrade module, acknowledging that it's OK to proceed without them?

Sample appearance: