yarn changeset should omit packages that aren't versioned in the prompt

[mtlewis]

Feature Suggestion

When running yarn changeset, it shouldn't be possible to select packages for which a changeset isn't valid. example-app is one such package, but there may be more.

Possible Implementation

Context

Omitting the packages that should never appear in a changeset will help people get changesets right with less human feedback needed.

[mtlewis]

Opened changesets/changesets#620 to request this feature in @changesets/cli!

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.