pkg/lib: add `#:versions?` argument to `extract-pkg-dependencies`
raco setup: check package version deps and multiple module declarations

The package dependency checking process was already gathering all the
reelvant information, or nearly all of it, so it's relatively cheap to
add the checks.
make pkg-links: shortcut for no-change case

Taking a shortcut skips dependency and module-declaration checks, but
that job is now covered by `raco setup`.
setup/getinfo: shortcut for simple modules

When an "info.rkt" module is simple enough, build a hash table
directly instead of compiling and loading the module.