If xref analysis is being run by a dependency during its compilation
phase, the xref provider will try to add that deps' parents to the check
job while the ebin/ directories for them do not exist. This causes a
crash.
This patch makes it so directories are only added if they are existing,
preventing failure on any compile hook for dependencies and allowing
successful compilation as a best effort.
Fred Hebert
9a6f04a2d9