Lookup leaf key methods in parents

Lookup leaf key methods in parents

Union is a valid key type and it can actually come from a grouping,
in which case it gets generated as an inner type. If that happens
the typedef is not populated into the usual places, leading to
a failure to find method return type -- leading to a missing key
member.

This is a problem similar to YANGTOOLS-424, so let's use the same
ModuleContext approach taken then.

JIRA: MDSAL-161
Change-Id: Id342701d603bdd2b9b78ba3295c8eae60f253429
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 6ac608d5c6b02fbcc3ee01af8fc4e3301a8ed52b)