Environment keys should not be case sensitive on Windows #134
Description
On Windows, the environment keys are case-insensitive. Instead of using a [String: String] type, we should define an EnvironmentKey structure that handles the case-insensitivity considerations across platforms.
This is rather important, as we've run into real bugs in the build system as a result of treating the environment as a case sensitive dictionary.
See https://github.com/swiftlang/swift-build/blob/main/Sources/SWBUtil/EnvironmentKey.swift for an example of how we did this in Swift Build.