i18n module: restore the <packagename>-gmo target

Removed in commit 487d45c1e5 but perhaps
it should not have -- people may have been depending on ensuring those
are built somehow. Even though the internal implementation changed and
it is now built by the all target, let's keep the old target around too.

Now it just aliases the actual build rules, though.

mesonbuild/modules/i18n.py

@@ -188,6 +188,9 @@ class I18nModule(ExtensionModule):
             targets.append(gmotarget)
             gmotargets.append(gmotarget)
 
+        allgmotarget = build.AliasTarget(packagename + '-gmo', gmotargets, state.subdir, state.subproject)
+        targets.append(allgmotarget)
+
         updatepoargs = state.environment.get_build_command() + ['--internal', 'gettext', 'update_po', pkg_arg]
         if lang_arg:
             updatepoargs.append(lang_arg)