Reorganize scripts modules to drop the try/except ImportError import shim

[toufali]

Several scripts (e.g. sweep.py) use a try/except ImportError shim to import shared modules under both scripts.<mod> (package) and <mod> (flat) runtimes:

```python
try:
from scripts.github_utils import SEVERITY_RANK, has_codeowner_approval, is_bot
except ImportError:
from github_utils import ... # type: ignore[no-redef]
```

This gets more cumbersome as shared modules grow. Reorganize the package layout / invocation so shared modules import cleanly in every runtime (workflow, CLI, tests) and the shim can be removed.

Raised by @groovecoder in review of #147.