[bk] Update individual package publish pipeline to handle rcs

[benpankow]

Summary

Updates the logic of our single-package publish BK pipeline to:

1. Allow selecting any package, not just those with fixed versions in the codebase
2. If a package is selected that doesn't have a fixed version in the codebase [meaning, it is a package released weekly with typical dagster release version numbers] mandate that the release version is a rc, so it doesn't interrupt typical user installs

The intent is that we can use this pipeline to (1) freeform release packages we don't regularly publish, e.g. dagster-airlift and (2) allow us to easily release release candidates for packages we do regularly publish, e.g. dagster-looker

Test Plan

Successful release:
https://buildkite.com/dagster/publish-single-python-package/builds/56#

Flagging non-rc version for traditional package:
https://buildkite.com/dagster/publish-single-python-package/builds/54

[benpankow]

• [bk] Update individual package publish pipeline to handle rcs #25352 👈
• master

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