Initialize package before calling get_loader() for __main__, so that we do
not incorrectly handle ImportError from __init__.py. When runpy is used from
the Python CLI, use an internal exception rather than ImportError, to avoid
catching an unexpected ImportError.

Also simplify message formatting: str() is redundant with %s.

Also fix test_dash_m_error_code_is_one() in test_cmd_line_script, which was
failing because the test package was not in the current directlry, rather
the desired ValueError.