build-aux/check-coupling: Prohibit supplier imports of UI packages

The reverse was checked, but apparently a check for suppliers importing the
UI was never added.

RELEASES.md

@@ -43,6 +43,11 @@ Linker
     entities within the XML document.  In some cases, it can reduce peak
     memory usage by half.
 
+Tools
+=====
+- `build-aux/check-coupling` will now prevent `supplier/` packages from
+  importing `ui/` packages; previously only the reverse was true.
+
 
 v18.0.3 (2021-07-21)
 ====================

build-aux/check-coupling

@@ -70,13 +70,13 @@ find-violations()
             $( non-supplier-maps ) \
     | grep ' /suppliers/'
 
-  # Suppliers must not import other suppliers.
+  # Suppliers must not import other suppliers or UI packages.
   # TODO: Check against supplier maps
   for supplier in suppliers/*.xml; do
     local name=$( basename "$supplier" .xml )
 
     lsimports "$supplier" $( supplier-packages "$name" ) \
-      | grep ' /suppliers/' \
+      | grep ' /suppliers/\|/ui/' \
       | grep -v " /suppliers/$name/"
   done
 }