bpo-45019: Clean up the frozen __hello__ module. (gh-28374)

Here's one more small cleanup that should have been in PR gh-28319. We eliminate stdout side-effects from importing the frozen __hello__ module, and update tests accordingly. We also move the module's source file into Lib/ from Toos/freeze/flag.py.

https://bugs.python.org/issue45019

Lib/__hello__.py

@@ -0,0 +1,7 @@
+initialized = True
+
+def main():
+    print("Hello world!")
+
+if __name__ == '__main__':
+    main()