Python 3 absolute imports not working after setup.py

Question

I have a Python 3 project I wanted to release. Here is the structure:

README.rst
setup.cfg
setup.py
projectname
├── __init__.py
├── __main__.py
├── file1.py
└── file2.py

In my setup.py I have the usual info (name, description etc.) and two relevant lines are:

packages=find_packages(),
entry_points={
    'console_scripts': [
        'project-name=projectname.__main__:main',
    ],
},

In my __main__.py, I do the following imports:

from file1 import a
from file2 import b

My project works fine when running __main__.py in the terminal normally:

python3 __main__.py

However, after running setup.py:

sudo python3 setup.py install

and running:

project-name

I get the following error:

ImportError: No module named 'file1'

What is strange is that if I do explicit relative imports instead:

from .file1 import a
from .file2 import b

This will work for the installed package but NOT when running __main__.py normally in the terminal where I will get the following error:

SystemError: Parent module '' not loaded, cannot perform relative import

I am completely lost here, can anybody shed some light? I have been reading on imports in Python 3 and nothing has helped me yet. Any help would be much appreciated. Please let me know if addition information is required. Thanks!