Imlement simple Plexus logger
In tools use simple logger
Simplify logging code a bit
Delay executing rpm until its output is really needed

If debugging is disabled then output of rpmquery will likely be
unneeded.  Delaying execution of rpmquery will prevent it from running
when it's not needed.

This change can improves running time of xmvn-resolve on my system by
half, from about 1.6 s to 0.8 s.
Improve logging messages about nonexistent files