refactor app discovery to stop depending on specific pnd_disco_t struct

Now functional with libpnd versions after b2aaaf39eef7bcc2bfa000fbdb9f72e6d0f656c8 (Patch from Cloudef to add very basic support for 'package' PXML tags).
Tested through current master (6f277b9a9e9b4b6a642bc1e82028d5394faaae49).