BUG-1537: improved YangModuleInfo.

YangModuleInfo is generated also for submodules in form as inner class in YangModuleInfo class of
module to which it belongs. Method YangModuleInfo.getImportedModules() returns also YangModuleInfo
classes of submodules which this module includes.
Added getSubmodules() method to Module interface.
Added test.

Change-Id: Id949835d960eee3197d249f7a83be2975a63d6b3
Signed-off-by: Martin Vitez <mvitez@cisco.com>
Merge "BUG-1537: improved YangModuleInfo."