Investigate changing `getVersionsForEnum` to not rely on namespace.

The `getVersionsForEnum` method has a reliance on namespace. In PR #1647 this was changed from a reliance on the enum's containing namespace to a reverse lookup of namespaces that version based on the enum, but there is concern that this could have unintended consequences.

The purpose of this issue it to investigate if we can remove the dependency on namespace entirely.

See:
https://github.com/microsoft/typespec/pull/1647#discussion_r1120464954