Import error on installed package using setup.py

You have to list all packages in setup, including subpackages:

setup(
    name = "mytestmodule",
    version = "0.0.1",
    description = ("A simple module."),
    packages=['mymodule', 'mymodule.subdir'],
)

Or you can use setuptools‘s magic function find_packages:

from setuptools import setup, find_packages
setup(
    name = "mytestmodule",
    version = "0.0.1",
    description = ("A simple module."),
    packages=find_packages(),
)

This is mentioned here:

If you have sub-packages, they must be explicitly listed in packages,
but any entries in package_dir automatically extend to sub-packages.
(In other words, the Distutils does not scan your source tree, trying
to figure out which directories correspond to Python packages by
looking for __init__.py files.)