x11/showmethekey: split CLI into subpackage

[jbeich]

Summary

Due to missing USE_*.<subpkg> helper and for POLA sake I've moved out CLI to help headless users. Turning main subpackage into meta-package to pull both cli and (in future) gui subpackages may be of dubious benefit.

Similar to D43581.

Test Plan

$ poudriere testport -j 132i386 x11/showmethekey~cli
[...]
===========================================================================
=>> Checking shared library dependencies
 0x00000001 NEEDED               Shared library: [libc.so.7]
 0x00000001 NEEDED               Shared library: [libevdev.so.2]
 0x00000001 NEEDED               Shared library: [libinput.so.10]
 0x00000001 NEEDED               Shared library: [libthr.so.3]
 0x00000001 NEEDED               Shared library: [libudev.so.0]

[jbeich]

Likely pointless as showmethekey-cli is similar to xkbcli interactive-evdev from libxkbcommon package. Besides,

$ showmethekey-cli -h
[...]
Warning: This is the backend and is not designed to run by users. You should run the frontend of Show Me The Key, and the frontend will run this.