Split commands into simpler commands

[joshuacurtiss]

Right now, some commands serve multiple "contexts" of Moodle maintenance. These are just suggestions currently, and not completely thought out/fully decided.

mdl copy

• mdl file copy
• mdl backup copy

mdl info

• mdl sys info
• mdl env info

mdl init

• mdl sys init
• mdl env init

mdl install-plugin

• mdl plugin install

mdl list

• mdl file list
• mdl backup list
• mdl env list

mdl remove

• mdl env remove
• mdl sys remove
• mdl backup remove
• mdl volume remove