Improve error message for invalid Python package (#20482)

Resolves #17108

Author: hauntsaninja

mypy/find_sources.py

@@ -195,7 +195,10 @@ def _crawl_up_helper(self, dir: str) -> tuple[str, str] | None:
             if not name.isidentifier():
                 # in most cases the directory name is invalid, we'll just stop crawling upwards
                 # but if there's an __init__.py in the directory, something is messed up
-                raise InvalidSourceList(f"{name} is not a valid Python package name")
+                raise InvalidSourceList(
+                    f"{name} contains {os.path.basename(init_file)} "
+                    "but is not a valid Python package name"
+                )
             # we're definitely a package, so we always return a non-None value
             mod_prefix, base_dir = self.crawl_up_dir(parent)
             return module_join(mod_prefix, name), base_dir

test-data/unit/cmdline.test

@@ -106,7 +106,7 @@ dir/a.py:1: error: Name "undef" is not defined
 [file dir/sub.pkg/a.py]
 undef
 [out]
-sub.pkg is not a valid Python package name
+sub.pkg contains __init__.py but is not a valid Python package name
 == Return code: 2
 
 [case testBadFileEncoding]