Use prefixed tags for publish config

[natebosch]

The auto-publish comment already indicates that the package name prefix
should be used.

[github-actions]

Package publishing

Package	Version	Status	Publish tag
package:source_gen	1.3.0	ready to publish (merge and tag to publish)	source_gen-v1.3.0

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[devoncarew]

We really only need to use prefix tags for packages in mono repos; v1.2.3 for single package repos, and my_name-v1.2.3 for packages in mono-repos (https://github.com/dart-lang/ecosystem/tree/main/pkgs/firehose#mono-repos).

[natebosch]

We really only need to use prefix tags for packages in mono repos

How does it pick the "Publish Tag' it adds to the comments - that is showing with the package prefix.

We've been using a package prefix in this repo for a while now, though I don't remember the reason why we started that pattern.

[devoncarew]

It figures out whether its in a mono repo or not, and expects one form of the tag or the other:

https://github.com/dart-lang/ecosystem/blob/main/pkgs/firehose/lib/src/repo.dart#L61

We can make this configurable if necessary - we'd just have to figure out the best way to do that (a yaml file in the repo? an arg to the publish task - https://github.com/dart-lang/source_gen/blob/master/.github/workflows/publish.yaml?)