Adds a new plugin, decoupled from `pyenv install`, for packaging and installing
relocatable Python binaries. This commit is just the command dispatcher; the
individual subcommands follow.
`pyenv binary <command>` dispatches to the matching script under the plugin's
libexec, so subcommands stay out of the top-level `pyenv commands` list.
Groundwork for the binary distribution support discussed in #2334.
