Add an option to allow package first publish

[C0Florent]

As of now, when trying to publish a package to a registry for the first time with melos publish, melos errors out when comparing the local version to the registry version of the package, even in dry-run mode. It makes sense that the comparison sort of fails because the package isn't in the registry yet.

melos publish does a great job at abstracting out individual package publication (the flutter/dart/pub publish commands), except in this particular case where we currently have to resort to manual publish commands. Would it be possible to add a boolean flag to allow melos to publish a package which is not yet in the registry? Something along the lines of --allow-first-publish for example