Test that package modules are accessible as symbols

We'd like to be able to access the package modules as symbols after importing the package's module (`__init__.py`).

I.e. given the `kcidb` package contains the `db` module, this should work:

```python
import kcidb
print(kcidb.db)
```
Most of the code is already built that way. However, we don't have any tests verifying that.

Add a test to `test_kcidb.py` which starts with the `kcidb` package, lists modules under it (e.g. using `pkgutil`, like in [`kcidb.monitor.subscriptions`](https://github.com/kernelci/kcidb/blob/main/kcidb/monitor/subscriptions/__init__.py)), imports the package and verifies that each listed module is available as a symbol with type `` inside it, then continues to all sub-packages recursively, until everything is verified.