Initialize endpoints through storage instead of computed property

[mbrandonw]

Right now something like this does not compile:

@DependencyClient private struct ClientWithClosuresBeforeNonClosures {
  var endpoint: () async throws -> Int  // 🛑 'self' used before all stored properties are initialized
  var identifier: String
}

The reason is that @DependencyEndpoint is applied to endpoint and not identifier, and that macro turns the stored property into a stored+computed property combo. The initializer sets only the computed property, not the stored. We used to work around this with an init accessor, but that caused other problems (#355). So instead we can just set the underscored stored property in the initializer.