BUG-997: Introduce ASTSchemaSource

ASTSchemaSource represents an intermediate processing step, where we
have parsed the YANG text and extracted dependency information.

Change-Id: Ice0b841404a28912787b83f88f9b2d28c68497d5
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-997: Introduce ASTSchemaSource"