Warn against publishing at version `0.0.x`

[natebosch]

The way pub handles 0.x.y is non-standard for semver and occasionally leads to confusion about how 0.0.x package versions behave.

There isn't any Dart-wide convention for what 0.0.x means, and it's safest to only publish 0.1.x and higher. We should consider adding a warning on publish for this. It would be safe to allow overriding it and publishing anyway.

[sigurdm]

How is 0.0.x different from 0.1.x. Do we have any conventions there?

is non-standard for semver and occasionally leads to confusion about how 0.0.x package versions behave.

Do we have concrete examples where this has caused confusion?

[natebosch]

How is 0.0.x different from 0.1.x. Do we have any conventions there?

No, we don't have conventions, but some users are mislead into thinking we do.

Our treatment of 0.x is non-standard for semver. In standard semver a bump from 0.x to 0.y is not a version incompatible change. In Dart ecosystem that is a version incompatible change. I've seen a few users get confused by that and think that 0.0.x to 0.0.y is also treated as an incompatible version bump by pub, but it's not.

There's no additional power or communicative intent behind 0.0.x vs 0.1.x, so we can close the gap that misleads some users without costing other users anything.