chore: fix #412 #622 iOS resolver reference when iOS module not installed

[StephenHodgson]

• fixes [BUG] Unable to resolve reference 'UnityEditor.iOS.Extensions.Xcode' on Windows #412
• fixes [Bug] IOSResolver.dll tries to load on Windows/Linux - causes error #622
• updated export_unity_package_config.json and separated Google.IOSResolver.* into its own PluginImporter section, adding defineConstraints key with UNITY_EDITOR and UNITY_IOS elements.
• lint export_unity_package_config.json with standard json lint, 2 spaces
• added defineConstraints to AssetConfiguration in export_unity_package.py
• added unit tests for defineConstraints in export_unity_package_test.py
• updated workflows to use latest version of checkout/setup actions

Fixes

Assembly 'Packages/com.google.external-dependency-manager/ExternalDependencyManager/Editor/1.2.183/Google.IOSResolver.dll' will not be loaded due to errors:
Unable to resolve reference 'UnityEditor.iOS.Extensions.Xcode'. Is the assembly missing or incompatible with the current platform?
Reference validation can be disabled in the Plugin Inspector.

Invalid when platform is not set to IOS or module not installed

Valid when platform is set to IOS

CC @a-maurice, @chkuang-g

[f1yingbanana]

This looks good, I'll try and get it reviewed :)

[a-maurice]

Thanks for sending this, and diving into the export_unity_package script! At a first glance it seems good, but I'll want to spend some time testing it with different install configurations to make sure it doesn't cause any issues, which might take a little bit of time. But I'll try to get to it when I can!

[StephenHodgson]

@a-maurice feel free to push any changes or remake this PR as needed :)