Remove mutable state from NativePlatform

In the new plugin binary types, we expose `NativePlatform` via `CppPlatform` and `SwiftPlatform`. These are intended to be immutable.

All of the existing native compilation/linking tasks use `NativePlatform` to identify the target machine's architecture and operating system. Internally, this type (or `NativePlatformInternal`) is used all over to select tool chains/tools. Effectively, the native platform needs to be immutable for this to all line up properly.

Because `NativePlatform` is also used in the software model for creating new target `Platform`s, it's mutable. We expose this mutability by reusing this type/interface.

We introduced a `OperatingSystemFamily` in 4.7 that carries some of the same information as `OperatingSystem` (a property of `NativePlatform`) and is used as an attribute in dependency management. This is an immutable property of `NativePlatform`.

Users should prefer the immutable `operatingSystemFamily` over `operatingSystem`, but we should also try to remove the mutability that exists.