Run distcc-pump server throughout src_configure() to src_install()

Start distcc-pump server in all phases throughout src_configure() to
src_install() rather than in src_compile() alone. Since each of those
phases may involve some compilations, we should try to take full
advantage of distcc in all of them. Moreover, this silences distcc
warnings about being unable to connect to the server when compiling.

Fixes: https://bugs.gentoo.org/show_bug.cgi?id=507482
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