In packages repo, swift-format should fall back to Xcode swift-format if not found on `PATH`

[jmagman]

Xcode 16 now packages swift-format

$ xcrun --find swift-format
Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-format

The format command should first check if --swift-format-path is passed in, then if swift-format is on the PATH, and then fallback to the results of the xcrun --find swift-format. And if that returns nothing, then fail.

https://github.com/flutter/packages/blob/1ab1a712e5df333026c9103fffa82f014e731d9b/script/tool/lib/src/format_command.dart#L230-L239