CommandInfo: Avoid loading delegate classes in incompatible  loaders

Some PluginInfo instances implement the Instantiable interface. These
instances should be allowed to return the delegate classes in their
favorite form.

A symptom of loading the delegate class in an incompatible class loader
is that gentyref's getExactFieldType() claims that a field is not a
member of its declaring class.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>