Log Message:
pbulk-base-0.56: Support for adjusting scheduling

Switch to a weighted scheduling algorithm. Before, build order was based
on number of reachable nodes in the dependency graph. This assumes that
heavy packages are required by other packages. Some big packages
nowadays violate that assumption and can result in long periods at the
end of a build where only a few builders are active. Annotating those
packages with PBULK_WEIGHT in the pbulk-index output can boost their
priority to let them be built earlier. The default weight is 100.

Note: the pbulk-build report has grown an extra field per line with the
computed effective weight of each package. This file is normally used
only internally.