pkg-push: use 'default in 'versions for current pkg info
move some test & doc collections out of "racket-" pkgs to new pkgs
raco setup: add --unused-pkg-deps flag

This flag may be useful for checking for unnecessary dependencies, but
beware that it reports many false negatives, either because relevant
files are not compiled, because the dependency is dynamic, or because
unused packages are intentially listed as dependencies as a
convenience (as is the case for "main-distribution").
remove some unneeded dependencies