*: Use relative Jsonnet imports internally

[maxbrunet]

• [x] I added CHANGELOG entry for this change.
• [x] Change is not relevant to the end user.

Changes

• Use relative Jsonnet imports internally
• Disable jb's legacy imports (not used, but making sure they are not accidentally used in the future)

Reasons:

1. There is no guarantee kube-thanos/ is a valid import path for the end-user
2. These imports were relying on jb's legacy imports which are deprecated

Verification

CI passes.

[maxbrunet]

@kakkoyun @metalmatze any concerns about merging this one if I rebase?