If at some point in the future we have another transition as extensive
as time64, then libgtk-3-0t64 could conceivably be replaced by some
other package, which I have modelled here as libgtk-3-0xyz. If that
happens, we need to avoid deletion of immodules.cache, otherwise we
will have another bug similar to #1065494.
This implementation is based on the assumption that third-party input
method modules for GTK 3 will depend on GTK 3, therefore we should not
need to clean up the IM modules cache unless/until we reach the point
of having no IM modules installed.
