Move ScriptToConstellationMsg to constellation_traits (#36364)

This is the last big change necessary to create the
`constellation_traits` crate. This moves the data structure for messages
that originate from the `ScriptThread` and are sent to the
`Contellation` to `constellation_traits`, effectively splitting
`script_traits` in half. Before, `script_traits` was responsible for
exposing the API of both the `ScriptThread` and the `Constellation` to
the rest of Servo.

- Data structures that are used by `ScriptToConstellationMsg` are moved
  to `constellation_traits`. The dependency graph looks a bit like this:
  `script_layout_interface` depends on `script_traits` depends on
  `constellation_traits` depends on `embedder_traits`.
- Data structures that are used in the embedding layer
  (`UntrustedNodeAddress`, `CompositorHitTestResult`, `TouchEventResult`
  and `AnimationState`) are moved to embedder_traits, to avoid a
  dependency cycle between `webrender_traits` and
  `constellation_traits`.
- Types dealing with MessagePorts and serialization are moved to
  `constellation_traits::message_port`.

Testing: This is covered by existing tests as it just moves types
around.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/navigation.rs

@@ -10,6 +10,7 @@ use std::cell::Cell;
 
 use base::cross_process_instant::CrossProcessInstant;
 use base::id::{BrowsingContextId, PipelineId, WebViewId};
+use constellation_traits::LoadData;
 use content_security_policy::Destination;
 use crossbeam_channel::Sender;
 use embedder_traits::ViewportDetails;
@@ -22,7 +23,7 @@ use net_traits::{
     BoxedFetchCallback, CoreResourceThread, DOCUMENT_ACCEPT_HEADER_VALUE, FetchResponseMsg,
     Metadata, fetch_async, set_default_accept_language,
 };
-use script_traits::{DocumentActivity, LoadData};
+use script_traits::DocumentActivity;
 use servo_url::{MutableOrigin, ServoUrl};
 
 use crate::fetch::FetchCanceller;