msgpack1: install in a dedicated prefix to avoid conflicts with msgpack, msgpack-c and msgpack-cpp

[barracuda156]

Error: Failed to activate msgpack1: Image error: /opt/local/include/msgpack.h is being used by the active msgpack-c port.  Please deactivate this port first, or use 'port -f activate msgpack1' to force the activation.
    while executing
"throw registry::image-error $msg"
    ("foreach" body line 49)
    invoked from within
"foreach file $imagefiles {
                incr progress_step
                _progress update $progress_step $progress_total_steps
                se..."
    invoked from within
"registry::write {
            # extract phase complete, assume 1/3 is done
            set progress_step $num_imagefiles
            foreach file $ima..."

[reneeotten (Renee Otten)]

by now most ports should hopefully have transitioned from these old msgpack versions. It doesn't look like there is any that depends on msgpack1 (or some of the other ones); so I would suggest to just get rid of them instead of spending time and effort.