Improve QNAME field definition

Improve QNAME field definition

The constant field that contains the QName corresponding to the
generated construct can rely on YangModuleInfo associated with
the generated module. Since that class already contains the module's
QName, we can lift it from there and use it to define the constant.

This has the benefit of not duplicating strings in classes, leading
to smaller footprint and improves startup time, as the strings do not
have to be parsed when the class is loaded.

JIRA: MDSAL-339
Change-Id: Ia7d336e3d386955f83648543b1e98f820ebbaf40
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>