Fix submodule resource resolution

Fix submodule resource resolution

Initial integration with yangtools-6.0.0 has changed the resource
resolution logic. Unfortunately the lambda instroduced in
Ifa9780fbdfb411b4613df0ee18407d85e81bd517 incorrectly passed 'module'
and not the requested entity to the resolver -- leading to all
submodules to open their module's resource instead.

Fix the lambda and also add proper asserts to check resources are being
open correctly.

JIRA: MDSAL-663
Change-Id: I293f20c15426490a97bb70b4303f0c7b23114c82
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 18ac80f3b2ff56bc734d236ba38d9cd14c5f06d7)