simplified namespace support, fixed ns sniff selection

Could not select sniffs with namespaces (only able to use all sniffs of namespaced standards).

Modifications:
* do only support namespaces after sniffs are filtered (wrong order previously)
* do keep listeners index namespace free to provide identical behaviour for namespaced and non-namespaced listeners (`listeners['Standard_Sniff_SomeSniff'] = 'Standard\Sniff\SomeSniff'`)
* simplify previous ns patch