pkg/dirs-catalog added

This utility that is needed by `make` turns out to be useful in other
scripts.
remove the "racket-pkgs" directory layer

The layer is now redundant, since everything left in "pkgs" is in the
"racket-pkgs" category.
fix handling of "links.rktd" errors
add makefile step to adjust for movements within "pkgs"

The step doesn't currently adapt to additionals or removals
from "pkgs", so further support may be needed in the future.