Derived types have been using their proper type for a long time now
and there is essentially cannot be a semantic difference between a
derived/restricted type which would make any difference during
resolution. Ditch the base type lookup and just use the type at hand.
JIRA: NETCONF-837
Change-Id: I21afc3ac15c954358f62070065531e75ff360924
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>