move everything into src/catnab and add more tests

pyproject.toml

@@ -17,7 +17,7 @@ dependencies = [
 ]
 
 [project.scripts]
-catnab = "main:main"
+catnab = "catnab:main"
 
 [dependency-groups]
 dev = [
@@ -25,8 +25,5 @@ dev = [
     "pytest>=8.4.2",
 ]
 
-[tool.pytest.ini_options]
-pythonpath = ["src"]
-
 [tool.uv]
 package = true