refactor(routes): move search domain into routes/search/ subpackage (#5779)

Slice 2j of the route-domain reorganization (#4082/#4071). Moves
search_routes.py into routes/search/, leaving a backward-compat
sys.modules shim. Pure file reorganization, no behavior change.

tests/test_search_routes_shim.py

@@ -0,0 +1,11 @@
+"""Regression test for the search route shim (slice 2j, #4082/#4071)."""
+
+import importlib
+
+import routes.search_routes as _shim_search  # noqa: F401
+
+
+def test_legacy_and_canonical_search_module_are_same_object():
+    legacy = importlib.import_module("routes.search_routes")
+    canonical = importlib.import_module("routes.search.search_routes")
+    assert legacy is canonical