sonic-yang-mgmt: Port code to new libyang3-py3

Changes:
 * general symbol names/prototype updates
 * find_data_dependencies() enhancements:
   There is logic in sonic-utilities to do multiple traversals of the
   data tree in order to come up with data dependencies.  This also
   calls into libyang directly.  Enhance logic so find_data_dependencies()
   can return the desired resultset with only a single traversal of the
   tree and remove dependence of libyang in sonic-utilities.

Author: bradh352