Currently if a runtime extension, e.g.
org.freedesktop.Platform.html5-codecs//18.08 is used by a runtime
org.kde.Platform//5.12 which itself is used by one or more apps, when we
print a message to the user about html5-codecs being EOL, we don't find
any apps using it and don't print any. Fix this by including apps that
indirectly use a runtime extension in the "Applications using this
runtime:" list.
In a later commit we can re-use the helper function added here to add a
confirmation dialog if the user tries to remove a runtime extension
that's being used; currently we just let them remove it.
This is limited to only looking in the current flatpak installation, so
a per-user app using a system-wide runtime extension would not be found.
This is implemented using in-memory caches because otherwise it is
horribly slow; see
https://github.com/flatpak/flatpak/pull/4835#discussion_r876425289
Helps: #3531
e739773056