Tests should import from the package, not internal modules

[dannywillems]

Problem

Tests import from internal modules instead of the public package API:

• `from leakix.client import Scope` instead of `from leakix import Scope`
• `from leakix.field import ...` instead of `from leakix import ...`
• `from leakix.plugin import Plugin` instead of `from leakix import Plugin`
• `from leakix.query import ...` instead of `from leakix import ...`
• `from leakix.response import ...` instead of `from leakix import ...`

This applies to `test_client.py`, `test_query.py`, and `test_response.py`.

Solution

• Change all test imports to use `from leakix import ...`
• This also validates that `init.py` correctly re-exports everything