Include more extensive dist-info for app package

**Is your feature request related to a problem? Please describe.**
Sometimes the main app also doubles as a regular Python package with a setup.py / setup.cfg which defines additional metadata. In some cases, the package may depend on having this metadata available at runtime. At the moment, briefcase will pip-install all dependencies but uses a minimal install step for the app itself which essentially takes the metadata from `pyproject.toml`.

**Describe the solution you'd like**
Would it be possible extend the app metadata for instance by pip-installing the app itself if a setup.py is found and then adding any additional briefcase-specific metadata afterwards? I realise that this may be a very niche issue and there could be good reasons against a pip-install (having less control over the install process, having possible console_script entry points installed, etc).

**Describe alternatives you've considered**
Keep the install process as-is and generate any additional dist-info "manually" from a setup.py if found.