Common code in local build package sources refactored out in a function; before building the real set of derivations needed is found (slightly slower for only one build strategy, but less garbage on output and better performance for multiple build strategies).
Now you have full choice of best-effort build regardless of method (substituters or actual build), using substituters, building only fixed derivations (should get you all the downloads) and local build without even trying substituters.
Some minor fix in the help text about behavior with no package sources.
