Fix transitive if-feature augments
We cannot just ignore augments, as they drive namespace population.
Instead of that, let's populate target nodes, but make sure they
are marked as unsupported.
This allows ChildSchemaNodeNamespace-driven lookups to find the resulting
node, but does not allow it to materialize as an EffectiveStatement.
JIRA: YANGTOOLS-859
Change-Id: Ib817e6e20563005427471098bf6ff2991fb60e01
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit
fdfe9c4914bf8a9f9a92cd1ab390e1f17a89b2ce)
(cherry picked from commit
b83966f5abb72503da77c19957c50f9e6dcd69be)