Fix DependencyResolver
We have a problem with submodules: a submodule cannot be resolved until
its belongs-to module is resolved -- but a module cannot be resolved
until its include submodules are resolved.
Fix this by partially restoring previous logic -- i.e. we consider
submodules resolved without considering belongs-to. We then add a
separate pass to invalidate resolved modules.
JIRA: YANGTOOLS-1572
Change-Id: I3b54912fe0b9ed86b98037b0ce4551a021663199
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>