Describe the bug
Running built AppImage loads Python sources from source tree instead of loading them from inside of the AppImage. This appears to happen when project's Python package is at project's root, and not in src folder.

You may get this abnormal folder structure if you create your project as Poetry project (with poetry new) and later on migrate to Briefcase.

To Reproduce
Steps to reproduce the behavior:

1. Create new Toga project using default answers with:

briefcase new
cd helloworld

2. Move helloworld package to root level:

mv src/helloworld .
rmdir src
sed -i 's|src/helloworld|helloworld|' pyproject.toml

3. Then build AppImage:

briefcase build

4. Run the built AppImage:

linux/Hello_World-0.0.1-x86_64.AppImage

5. You should see this:
   
6. Next in helloworld/app.py change window title from:

self.main_window = toga.MainWindow(title=self.formal_name)

To:

self.main_window = toga.MainWindow(title='You cannot see me!')

7. Finally run the AppImage again. Do not build it, just run the previously built AppImage:

linux/Hello_World-0.0.1-x86_64.AppImage

8. The title has changed, even though it shouldn't have:
   

Expected behavior
The title should have remained as "Hello World" as I didn't rebuild the AppImage.

Environment:

• Operating System: Pop!_OS 20.04
• Python version: 3.8.10
• Software versions:
  • Briefcase: 0.3.5
  • Toga: 0.3.0.dev30

Additional context
This bug appears to depend on current working directory. If you cd to parent directory then the AppImage will load its sources correctly from inside of it:

cd ..
helloworld/linux/Hello_World-0.0.1-x86_64.AppImage