[BUG] Buf registry packages have broken imports #65

Description

Relative imports are broken when a Buf module has a dependency on another one and you try to import a protobuf type.

Steps to Reproduce

1. Create two Buf modules
2. Add a dependency from one to the other
3. Install the package that has the dependency
4. Try to import a protobuf type

(You can also depend on a pre-existing module, e.g. protovalidate)

Expected Behavior

Package is installable and types are importable.

Actual Behavior

The Buf registry packages use relative imports that are incorrect and try to recurse too many levels upwards.

Screenshots/Logs

from ...buf.validate import validate_pb
ImportError: attempted relative import beyond top-level package

Environment

• protobuf-py Version: 0.2.0 / 0.3.0
• protobuf-py Plugin (protoc-gen-py) Version: Unsure - whatever Buf registry is using
• Python Version: 3.12
• Protobuf Compiler & Version: Unsure - whatever Buf registry is using
• Operating System and Version: macOS 26.6.1 (aarch64)

Possible Solution

Additional Context

See #15