briefcase dev fails if an application puts it's application module in the project root

[freakboy3742]

If an application places its source code directly in the project root, briefcase dev fails to start.

To Reproduce
Steps to reproduce the behavior:

1. Run Tutorial 0 in the Toga code repository using briefcase dev
2. See error

[tutorial] Starting in dev mode...
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/runpy.py", line 205, in run_module
    mod_name, mod_spec, code = _get_module_details(mod_name)
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/runpy.py", line 140, in _get_module_details
    raise error("No module named %s" % mod_name)
ImportError: No module named tutorial

Unable to start application 'tutorial'

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
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Environment:

• Operating System: macOS
• Python version: 0.3.10
• Software versions:
  • Briefcase: 0.3.9

Additional context

This appears to be an unintended side effect of #803 removing the current working directory from the PYTHONPATH when running briefcase dev