Modify handling of platform site to allow for venv usage

[freakboy3742]

Modifies how the support package handles the "platform site".

Previously, the platform site was a location that could be placed on the PYTHONPATH to make an environment appear to be an iOS environment. It uses a siteconfigure.py file to achieve this.

This PR preserves this usage; but also adds a make_cross_venv.py script. If you invoke this script, providing the location of a macOS virtual environment, that virtual environment will be modified (by adding some files to the site packages folder) in such a way that the virtual environment will appear to be an iOS environment. In addition, if build is invoked in that environment, the isolated venv created as part of that build will also appear as an iOS environment.

The location of the folder has been moved into the XCframework itself, so that the framework slices have readily access to the platform configuration for that slice of the framework. The names have also been modified to use the multiarch naming for each slice, rather than a name derived from the makefile naming conventions.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Currently failing CI because beeware/briefcase#2155 is required to adapt to the iOS platform-config changes.