init: Keep parent groups in addon comps environments

The environment in comps for a variant can refer to groups in parent
variant (either for addons, or because of other configuration). We
should not remove the groups in this case.

This requires changes in two places:
 * teaching `comps_filter` about groups that should not be removed
 * fixing writing comps so that it does not actually change the data as
   well

JIRA: COMPOSE-2612
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/data/dummy-comps.xml

@@ -135,6 +135,17 @@
     </grouplist>
   </environment>
 
+  <environment>
+    <id>foobar</id>
+    <name>Foo Bar</name>
+    <description>Referencing a group from parent variant</description>
+    <display_order>10</display_order>
+    <grouplist>
+      <groupid>resilient-storage</groupid>
+      <groupid>text-internet</groupid>
+    </grouplist>
+  </environment>
+
   <!-- LANGPACKS -->
 
   <langpacks>