deploy: Add optional 'check' extensions

A write extension will have various kinds of sanity checks to do before
actually performing the write. The current architecture of 'morph
deploy' means that several minutes pass between the user starting the
command and the write extension actually executing. It would be
rage-inducing watching `morph deploy` spend 3 minutes unpacking a
system only to then abort due to a silly error such as forgetting the
--upgrade switch. Therefore it's better for now to split the sanity
checks out into separate extensions that can be run as soon as possible
and abort if the write extension is not going to be able to operate.

For now this will just be used to validate usage of the --upgrade flag
but in future checking connectivity to remote servers and the like
should be done here too.

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