Is it possible that PrimeFaces is defining the taglib via annotations directly in Java classes or is it generating it during runtime? [...] the javaeetutorial is not describing any other opportunity than defining a ...-taglib.xml
As PrimeFaces 4 is JSF 2.2 compatible it could theoretically have done just that. Namely, in JSF 2.2/Java EE 7 it's indeed possible to define a Facelets tag directly via an annotation. For more details about this see: http://arjan-tijms.omnifaces.org/p/jsf-22.html#594
Since PrimeFaces 4 is still JSF 2.1/2.0 compatible it seems they haven't done it yet as you discovered in the meantime.