core: better support for ad-hoc configs

properly reload/unload the relevant modules so hopefully no more weird hacks should be required

relevant
- https://github.com/karlicoss/promnesia/issues/340
- https://github.com/karlicoss/HPI/issues/46

my/simple.py

@@ -0,0 +1,21 @@
+'''
+Just a demo module for testing and documentation purposes
+'''
+from dataclasses import dataclass
+from typing import Iterator
+
+from my.core import make_config
+
+from my.config import simple as user_config
+
+
+@dataclass
+class simple(user_config):
+    count: int
+
+
+config = make_config(simple)
+
+
+def items() -> Iterator[int]:
+    yield from range(config.count)