_backtrack_depgraph: fix bug #523048

This fixes _backtrack_depgraph to immediately report necessary
REQUIRED_USE changes instead of discarding the graph. This is
accomplished by replacing the depgraph.success_without_autounmask
method with a new need_config_change method that accounts for both
autounmask and REQUIRED_USE changes.

X-Gentoo-Bug: 523048
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=523048