Modify pip handling to use platform tag for iOS.

[freakboy3742]

With the release of pip 24.3, we're now in a position to use explicit --platform tags when installing wheels for iOS.

This removes the need for the platform-site shim to "fake" pip installation; we can instead rely on default pip/Python platform handling.

As part of this change, the use of wheels is now required, through use of the --only-binary=:all:. This is required as part of using --platform, as pip can't install from source when installing onto a foreign target platform, even if the wheel being installed is pure Python. There is a small minority of packages that are only packaged as source tarballs; but the broader Python ecosystem is firmly promoting the use of wheels, so this seems like a good opportunity to force the issue.

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

[mhsmith]

What's the situation with environment markers, like this line in the toga meta-package's pyproject.toml file?

    "toga-iOS == {version}; sys_platform=='ios'",

As I understand it, these aren't affected by the --platform option, and in fact are an open question in pip itself:

• Determining cross-platform resolution strategy pypa/pip#13111
• Various package-index filtering flags do not affect the environment markers pypa/pip#11664

I patched Chaquopy's copy of pip to deal with this, though at that time sys.platform and platform.system returned "linux" on Android:

• https://github.com/chaquo/chaquopy/blob/16.0.0/product/gradle-plugin/src/main/python/pip/_internal/cli/cmdoptions.py#L557
• https://github.com/chaquo/chaquopy/blob/16.0.0/product/gradle-plugin/src/main/python/pip/_vendor/packaging/markers.py#L291

[freakboy3742]

That's a good point - looks like we can't abandon the platform site package after all. --platform will guarantee a matching binary wheel - but we still need a cross-platform-enabled pip to do the install. We had a couple of failures in the dependabot updates because the testbed uses platform markers to prevent the installation of numpy etc.

Looking at the bigger picture - I wonder if it would be possible to use _PYTHON_HOST_PLATFORM and _PYTHON_SYSCONFIGDATA_PATH as a configuration point, store the host overrides in the platform-specific sysconfig, and have them incorporated into pip (or anything else that needs it) as required...