Implemented module dependency sort, that returns modules in order in which they shoul...

Implemented module dependency sort, that returns modules in order in which they should be processed.
If module A imports module B, sort returns ordered list [B,A].

Implemented utility topological sort that is used by module dependency sort.
Implemented tests for both topological sort and module sort.

Removed circular dependencies from test yang files.

Change-Id: Id6230a8448b27f80146dc74bf2a8a948523408d3
Signed-off-by: Maros Marsalek <mmarsale@cisco.com>