Allow Errata 5617 expressions in leafref

This updates PathExpression interface to allow expressing
leafref paths which start with a deref(...) function call,
as proposed by https://www.rfc-editor.org/errata/eid5617.

This unfortunately necessitates breaking the API contract of this
interface, as the original object model could not express these.

JIRA: YANGTOOLS-968
Change-Id: I3a193d338de5a82b95d2dd6cc5200e77032e67d9
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
Signed-off-by: Peter Suna <peter.suna@pantheon.tech>