[UX] Add information about removed modules to the uninstall page

Though we won't be able to actually do any uninstalling -- because we need the module to do that -- we can at least let the user know that something needs to be uninstalled.

If there is an entry in the file system for a module that is disabled (or enabled but removed) I'd like to show information on the Uninstall modules page prompting the user to add back the missing code, and uninstall properly.

It could be tricky to determine which modules need to be uninstalled (and which don't) but I think it's a worthwhile effort to think about how we might do that.

We could even (possibly?) use the schema_version to determine which version of the module to recommend.

This was spun-off from https://github.com/backdrop/backdrop-issues/issues/541