Move all supported r-spatial packages to Suggests

[Aariq]

In #4 @brownag mentioned moving terra and other r-spatial packages from Imports to Suggests, and I think this is a good call. We shouldn't require users to have all supported r-spatial packages installed just to use targets with one of them. rlang::check_installed() prompts users to install a missing package and installs it for them, but since this code will mostly be run in a callr environment, it might be more appropriate to just use rlang::is_installed(). We could even supply a custom tip like

! package terra is required.
Did you forget to add "terra" to tar_option_set(packages = ?