Flatten submodule hierarchy for YangModuleInfo

Flatten submodule hierarchy for YangModuleInfo

YANG inherently contains modules and submodules, where submodules
do not really have further submodules. While YANG 1.0 allowed
inclusion of submodules in other submodules, and that is modeled
in yang-model-api, this practice has been deprecated with YANG 1.1.

If we are faced with multiple such includes, for example both
module and submodule including a second submodule, we could end up
generating superfluous classes, just to represent the second
submodule twice (once for module's purposes and once for submodule's
purposes) -- hence when all of them are collected recursively, we
would end up with two distinct instances for the same source.

Since submodules are not directly visible outside of the main class,
this hierarchy is not public API and is a purely internal thing.

This patch re-organizes generation so that we present all submodules
as dependencies of the main module, which automatically solves the
problem, while also making for nicer class layout, as we do not get
multiply-nested classes.

JIRA: MDSAL-457
Change-Id: Ic78edaf852f3497f3c4c96fdca3a77de30b7f8c1
Signed-off-by: Anna Bencurova <Anna.Bencurova@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>