pkg-directory: add `#:cache` argument

The cache enables multiple calls to `pkg-directory` to load
installed-package information only once.
makefile: remove lingering references to special native packages
package "info.rkt": define `distribution-preference`

For now, the client is "distro-build".
make: always `raco setup` in default target

Use `--no-setup` when installing packages to avoid a
definitely-reundant setup.