entry point script problem with pyproject.toml

Question

I'm trying to create a python program that has an entry point script in pyproject.toml. However, I haven't been able to get the program to execute from both the entry point script for the installed package in a venv, and from the command line in the usual manner. The only way I could get it to work was to have two copies of the starting module file, in separate directories.

The directory structure is:

/path/to/example/project/
    ├── example/                    Python package directory.
    │   ├── __init__.py             This makes the directory a package.
    │   ├── start_module.py         The starting program module 
    │   └── main_module.py          The main program module.
    ├── start_module.py             Copy of the starting program module.
    ├── pyproject.toml              Definition of build process of the package.
    └── setup.cfg                   Configuration details of the python package.

and the pyproject.toml has:

[project.scripts]
runscript = "example.start_module:main"

start_module.py consists of:

from example.main_module import main_Module
def main():
    result = main_Module()

I can run this program from the command line in root directory project/ using:

python start_module.py

and from the virtual environment where the package is installed using:

runscript

but I haven't been able to reconcile these two arrangements with a single copy of start_module.py in either the root directory or the package directory. I think I'm missing something about package imports but can't put my finger on it.