Duplicate packageName instances in GeneratedTOBuilderImpl

A heap dump analysis shows 1093 duplicate Strings, wasting 190kB, all containing:

"org.opendaylight.yang.gen.v1.urn.ietf.params.xml.ns.yang.iana._if.type.rev140508"

which are retained from GeneratedTOBuilderImpl and its inner classes.

This looks like a failure to share the package name string across objects generated for a single model. Investigate ways how we can share instances to eliminate these duplicates.

A quick grep through sources seems to point to BindingGeneratorUtil's packageNameForTypeDefinition() and packageNameForGeneratedType(). Both these functions look like they may end up needlessly duplicating an input string.