Use MarkerRefs to resolve landing symbols in Mercurial

Summary: Ref T13546. Update the Mercurial code which finds default targets and maps symbols to targets under "arc land" to use the new MarkerRef workflow.

Test Plan: Ran "arc land" with (and without) various arguments in Mercurial, saw them resolve in a seemingly sensible way.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21335