[ecosystem] Autopublisher should take SDK constraint into account #195

The publisher can be configured to use a different channel as a repo-wide setting: https://github.com/dart-lang/ecosystem/blob/main/.github/workflows/publish.yaml#L60-L67

This has multiple downsides:

• It doesn't enable different packages in a mono-repo to target different SDK versions
• Devs can forget to update the channel setting when changing the SDK constraint between stable and dev/beta.

Related discussion: dart-lang/native#186