Reduce TestProfile mutable API: uploaders can be inferred directly from the profile.

[isoos]

• Follow-up to TestProfile packages: explicit imported and generated fields. #8634.
• The update-only mutability of TestPackage makes it harder to subclass it (especially if the TestVersion is also going to subclassed). Since we can infer the same default uploader from the TestProfile during import, there is no need to update it.
• Note: I'm planning to also remove the versions update in a subsequent PR.