Optimize merge order to try and select nodes that only have

Optimize merge order to try and select nodes that only have
unsatisfied PDEPEND slightly earlier. This solves a problem
with xorg-server being merged too early during an all binary
install (since DEPEND is ignored for binaries), triggering
built_with_use() calls to fail as reported in bug #189966.
Since DEPEND is discarded in cases like this, it is
important to exploit the difference between PDEPEND and
RDEPEND in order to optimize merge order. Without this
optimization, the merge order is technically correct, but
not as optimal as it should be and has lots of potential to
trigger issues with built_with_use() or similar things that
require better optimization of merge order.

svn path=/main/trunk/; revision=8358