BUG-1210: refactored imports handling in parser.

BUG-1210: refactored imports handling in parser.

Currently builder for module has no information about imported modules. This causes overhead because before resolving any of imported nodes
(uses, typedef, identities, augment path, deviation path...) parser needs to search whole collection of available modules to find target
module of imported node.

This patch adds information about imported modules to ModuleBuilder, so they can be available without search. Searching is performed only
once when resolving imports.

Change-Id: Ifcd4e995431165820c142e486da0466edc6abca0
Signed-off-by: Martin Vitez <mvitez@cisco.com>