This fixes a problem introduced in #235 that can cause dependencies to not be cached when first accessed in a re-entrant fashion. This can happen if while resolving a dependency one synchronously accesses another dependency:

struct OuterDependency: TestDependencyKey {
  static var testValue: OuterDependency {
    @Dependency(InnerDependency.self) var innerDependency
    _ = innerDependency
    return Self()
  }
}

The act of touching innerDependency synchronously while resolving OuterDependency causes re-entrancy into the cache of dependencies. And because #235 started protecting that cache with LockIsolated.withValue { … }, that causes the resolution of innerDependency to not be cached.

For now we will go back to protecting this state with a separate lock + mutable value, but in the future we may consider not allowing this kind of re-entrance into the cache. I think it's very subtle, and there are other ways of accomplishing this while avoiding re-entrancy.