Reshuffle include/ directory layout

This CL reorganizes the include/ directory as follows:
1. The immediate subdirectories of include/ are now the stable API
   surface exposed to all embedders, with the exception of ext/.
2. ext/ contains the unstable API surface that is exposed only to chrome.

Test: builds on all configurations and passes tests
Bug: 132678367
Change-Id: If638d4b4c150fd591cd1db851283b25ff6083ed6
diff --git a/test/fake_producer.h b/test/fake_producer.h
index 460d88c..4fddb8e 100644
--- a/test/fake_producer.h
+++ b/test/fake_producer.h
@@ -21,11 +21,11 @@
 #include <random>
 #include <string>
 
-#include "perfetto/base/thread_checker.h"
-#include "perfetto/tracing/core/data_source_descriptor.h"
-#include "perfetto/tracing/core/producer.h"
-#include "perfetto/tracing/core/trace_config.h"
-#include "perfetto/tracing/ipc/producer_ipc_client.h"
+#include "perfetto/ext/base/thread_checker.h"
+#include "perfetto/ext/tracing/core/data_source_descriptor.h"
+#include "perfetto/ext/tracing/core/producer.h"
+#include "perfetto/ext/tracing/core/trace_config.h"
+#include "perfetto/ext/tracing/ipc/producer_ipc_client.h"
 #include "src/base/test/test_task_runner.h"
 
 namespace perfetto {