textproc/py-CommonMark: Allow installation with textproc/cmark

Both ports install bin/cmark. Users of textproc/py-CommonMark are
supposely more interested in the python module rather than the command
line tool: nothing in the ports tree seems to use bin/cmark from this
port.

Rename the command to cmark-py to fix conflict

PR: 274848
Reported by: jcfyecrayz@liamekaens.com