This is my second try at this question. 

I am attempting to build a python project for distribution and I am having trouble getting imports of modules in my project to work in both development and the build release. Here is a simple example which shows the issue I am having.

Given a directory structure as follows:

BuildTest:
 -> pyproject.toml
 -> src
     -> zero
	 -> add.py
	 -> sub.py
	 -> zero.py
 -> test
    empty folder

Contents of pyproject.toml:

[build-system]
requires = ["setuptools", "setuptools-scm"]
build-backend = "setuptools.build_meta"

[project]
name = "zero"
version = "1.0"
description = "No matter the value it is zero"
authors = [{ name = "Confused Dev", email = "CD@dumb.com" }]
license = {text = "MIT"}
readme = "README.md"
requires-python = ">=3.11"
dependencies = []

[project.scripts]
zero = "zero.zero:main"

[project.optional-dependencies]
dev = []

Contents of src/zero/add.py:

def add(x, y):
    """Add two numbers together."""
    return x + y

Contents of src/zero/sub.py:

def sub(x, y):
    """Subtract two numbers."""
    return x - y

Contents of src/zero/zero.py:

import sys

# these imports work for installed version
import zero.add as add
import zero.sub as sub

# these imports work for local development version
# import add as add
# import sub as sub

value = 0

def main(args=None):

    value = add.add(0, 10)
    value = sub.sub(value, 10)
    print(f"WTF? The value is {value}")

if __name__ == "__main__":
    sys.exit(main())

If I build this project using python -m build, it builds a .whl file which I can then install and when I run 'zero' it works as expected.

However, if I then go to the src/zero directory and run:

python zero.py

I get the following error:

Traceback (most recent call last):
  File "/Users/tbrown/Library/CloudStorage/Dropbox/Docs/projects/build_test/src/zero/zero.py", line 2, in <module>
    import zero.add as add
  File "/Users/tbrown/Library/CloudStorage/Dropbox/Docs/projects/build_test/src/zero/zero.py", line 2, in <module>
    import zero.add as add
ModuleNotFoundError: No module named 'zero.add'; 'zero' is not a package

If I instead use the following for imports:

import add as add
import sub as sub

It works as expected in the local directory. However, if I build with these imports and install, then the installed version gives me the same import errors about finding functions add and sub.

So the question is how do I structure the project with import code that works both when built and installed, and locally in the development directory? What am I missing?

I have hacked at this for a while with no success and searching the internet for the problem has only confused me.