Obey generics in plugin collection parameters when populating them

Right now, when determining the target type for a collection item to convert POM configuration to, Maven/Plexus uses some heuristics based on the XML element name and the mojo package name and eventually falls back to String. The current approach requires plugin authors to either put their parameter bean classes into the same package as the mojo or requires users to explicitly use the implementation attribute in the POM. These troubles can be avoided if Maven would recognize the generic type of a parameter declaration List<Bean>, thereby aligning collections with arrays regarding the ease of configuration/use.