Implement census analysis: read, count, summarise, write and CLI

Closes #11
Closes #12
Closes #13
Closes #14
Closes #15

Task: MIL-002#4
Task: MIL-002#5
Task: MIL-002#6
Task: MIL-002#7
Task: MIL-002#8

tests/test_imports.py

@@ -0,0 +1,40 @@
+"""Every module of the package imports only the standard library or itself."""
+
+import ast
+import pathlib
+import sys
+
+import squirrel
+
+PACKAGE_NAME = squirrel.__name__
+PACKAGE_DIR = pathlib.Path(squirrel.__file__).parent
+ALLOWED = sys.stdlib_module_names | {PACKAGE_NAME}
+
+
+def _imported_top_level_names(source: str) -> set[str]:
+    imported: set[str] = set()
+    for node in ast.walk(ast.parse(source)):
+        if isinstance(node, ast.Import):
+            imported.update(alias.name.split(".")[0] for alias in node.names)
+        elif (
+            isinstance(node, ast.ImportFrom)
+            and node.module is not None
+            and node.level == 0
+        ):
+            imported.add(node.module.split(".")[0])
+    return imported
+
+
+def test_package_has_modules_to_check() -> None:
+    assert list(PACKAGE_DIR.glob("*.py"))
+
+
+def test_every_module_imports_only_standard_library_or_squirrel() -> None:
+    offenders: dict[str, set[str]] = {}
+    for module in PACKAGE_DIR.rglob("*.py"):
+        source = module.read_text(encoding="utf-8")
+        foreign = _imported_top_level_names(source) - ALLOWED
+        if foreign:
+            offenders[module.name] = foreign
+
+    assert offenders == {}