Ensure multi-tagged wheels are correctly identified as universal

[freakboy3742]

If a macOS binary wheel has multiple platform tags, Briefcase's platform merging code won't correctly identify this fact, and will mark the wheel as requiring merging. This is because the WHEEL metadata is a multi-valued header - it contains an independent Tag entry for each tag in the wheel.

The example package that revealed this was orjson, which is tagged cp313-cp313-macosx_10_15_x86_64.macosx_11_0_arm64.macosx_10_15_universal2.whl. The wheel metadata for this wheel reads:

Wheel-Version: 1.0
Generator: maturin (1.11.5)
Root-Is-Purelib: false
Tag: cp313-cp313-macosx_10_15_x86_64
Tag: cp313-cp313-macosx_11_0_arm64
Tag: cp313-cp313-macosx_10_15_universal2

Briefcase identifies the tag for this wheel as arm64, because this is the value for the Tag header that is returned from the WHEEL file (presumably because it is alphabetically last). This means Briefcase doesn't identify the wheel as universal, and so it installs a second copy of the same wheel as the "other" architecture, and then fails on merging because there is an existing x86-64 slice in the binaries being merged.

This PR adds multi-tag handling to WHEEL file processing, ensuring that any universal2 entry is identified as a universal wheel. It also ensures that if a wheel is multi-tagged as arm64 and x86-64, that is also identified as effectively universal (as no merging is required).

Fixes #2690.

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