Rename `esmvaltool_commands` entry points group name

[bouweandela]

We might want to change the name for the entry points group to esmvaltool.commands. That would be more in line with the specification and prepare us for possible future entry points esmvaltool.recipes and esmvaltool.diagnostics.

Originally posted by @zklaus in #2096 (review)

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[bouweandela]

This would be nice, but not following the specification seems unlikely to cause conflicts as long as no-one decides to start a package called esmvaltool_commands.