[Feature Request] Warn on overly general version spec

[hugovincent]

Best practice for yotta modules is to use appropriately specific dependencies version spec, so that users of your module don't experience breakage when your dependencies get updated -- but several of (our) yotta modules are still using wildcard specifiers (example). It would make sense for yotta to warn you before publishing the module if you have such a version spec. See also this explanation on handling circular references using wildcards. cc @jaustin