Rework make part: use include rather than recursive makes.

(+):
- we do not walk common dependencies many times

(-):
- we now need a wrapper around first make call
  to do cleanup (as the main Makefile is re-read,
  cleanup tied to Makefile will also be performed
  several times in middle of execution)
- overall reduction of robustness
Move some internal scripts to utils/ directory.
Add prefetch-existing.sh (mostly debug means).
Fix update-all.sh to reflect new makefile structure.
Fix dependency record generation: add to PKGDEPS array.
Improve gc.sh (subroutine aur): search by PKGBUILD, not by "src" folder.
Exit makefile generation early if PKGBUILD is not found.

This is needed to have make output meaningful errors
(i. e. say name of target which needs the failed dep).