depgraph._add_dep: fix bug #520950

This handles a case which occurs when
_solve_non_slot_operator_slot_conflicts calls _create_graph. In this
case, ignore unsatisfied deps for installed packages only if their depth
is beyond the depth requested by the user and the dep was initially
unsatisfied (not broken by a slot conflict in the current graph).

Since depth is meaningless for packages that are not reachable as deep
dependencies of arguments, the _UNREACHABLE_DEPTH constant is used as
the depth value for any packages added via _complete_graph. Also, any
sets added via _complete_graph have their reset_depth attribute set to
False.

The sys.stderr -> writemsg changes are necessary to ensure that the test
cases do not output unwanted error messages.

X-Gentoo-Bug: 520950
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=520950