Add @NonNull annotation to $YangModuleInfoImpl.qnameOf()

Add @NonNull annotation to $YangModuleInfoImpl.qnameOf()

YangModuleInfoTemplate has a lot of ceremony when we statically
know the types being used by the template. Concentrate imports
into two sets, instantiate them as strings and be done with it.

Since this takes care of most dynamics, making things much easier
and predictable, we can easily add @NonNull annotations as needed.

JIRA: MDSAL-487
Change-Id: Ic9cd4da1002f3edea6f3fb18f9d49e4a377e145b
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit b0ea6f0f70fbc597779703a4c65dd85b69355de3)