Fix packages tab completion (!1731)

Make tab completion work again for packages, if the default pmaports
dir is used. In the last "args" code refactor, the variable replacing
code was moved into its own function. We did not call it in the
packagecompleter() yet, so it could not replace the $WORK variable in
the default pmaports path.

pmb/parse/arguments.py

@@ -252,6 +252,7 @@ def arguments_repo_missing(subparser):
 def packagecompleter(prefix, action, parser, parsed_args):
     args = parsed_args
     pmb.config.merge_with_args(args)
+    pmb.helpers.args.replace_variables(args)
     packages = set(pmb.helpers.pmaports.get_list(args))
     return packages