packaged app: use executable named for the project and main function (#7670)
## Summary
Create a function main as the default for a packaged app. Configure the
default executable as:
`example-packaged-app = "example_packaged_app:main"`
Which is often what you want - the executable has the same name as the
app.
The purpose is to more often hit what the user wants, so they don't have
to even rename the command to start developing.
## Test Plan
- existing tests are updated