d/libgtk-3-0t64.postrm: Avoid recurrence of #1065494 in the future

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.

debian/libgtk-3-0t64.postrm.in

@@ -6,8 +6,8 @@ set -e
 
 case "$1" in
     (purge)
-        if [ -d /@MODULES_BASE_PATH@ ]; then
-            # Purge the cache
+        if [ -d /@MODULES_BASE_PATH@ ] && ! [ -e /@MODULES_BASE_PATH@/immodules ]; then
+            # Purge the cache, but only do this if no more IM modules exist.
             rm -f /@MODULES_BASE_PATH@/immodules.cache
             rmdir -p --ignore-fail-on-non-empty /@MODULES_BASE_PATH@
         fi