BUG-1070: split off YangSourceContext

This patch just splits up the various static classes out of
YangSourceContext, making the structure more navigable.

Change-Id: I26c973ebe6d931d870e22d5965ba7baab0cc24e9
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1070: split off YangSourceContext"