Restconf fails to deserialize URL if identityref from external module is present

Restconf is using global schema context to deserialize external identityrefs.

For urls such as

http://localhost:8181/restconf/config/network-topology:network-topology/topology/topology-netconf/node/vpp/yang-ext:mount/ietf-access-control-list:access-lists/acl/vpp-acl:vpp-macip-acl/macip

where vpp-acl and dhcp are external modules, restconf fails with message:

vpp-acl:vpp-macip-acl from URI can't be resolved. For key which is of type identityref it should be in format module_name:identity_name.

Using schema context provided by mount point, fixes the issue.