The plugin and plugins command had a name that was close, and while
plugin is not supposed to be directly called by Packer users, this could
happen by accident while trying to execute packer plugins subcommands,
and when it does, the error messages are far from explicit, so unless
they understand what Packer is doing here, they'll likely be lost.
To reduce the risk of confusion, we rename the command to run packer
embedded components as execute.
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