DevTools: Move Source related code to dedicated source.rs file (#36667)

Currently Source related code exists in watcher.rs and thread.rs. This
change moves source-related code to a dedicated source.rs file. This is
in preparation for adding support for showing source code in the
Debugger > Source panel.

- [x] Testing: These changes should not affect current functionality as
it only moves the existing code
- [x] Fixes: part of https://github.com/servo/servo/issues/36027

---------

Signed-off-by: atbrakhi <atbrakhi@igalia.com>

components/devtools/actors/watcher.rs

@@ -265,7 +265,7 @@ impl Actor for WatcherActor {
                         },
                         "source" => {
                             let thread_actor = registry.find::<ThreadActor>(&target.thread);
-                            let sources = thread_actor.sources();
+                            let sources = thread_actor.source_manager.sources();
                             target.resources_available(sources.iter().collect(), "source".into());
                         },
                         "console-message" | "error-message" => {},