Split payload application code into a subdirectory.

This patch splits from the main libupdate_engine code the part that
is strictly used to download and apply a payload into a new static
library, moving the code to subdirectories. The new library is divided
in two subdirectories: common/ and payload_consumer/, and should not
depend on other update_engine files outside those two subdirectories.
The main difference between those two is that the common/ tools are more
generic and not tied to the payload consumer process, but otherwise they
are both compiled together.

There are still dependencies from the new libpayload_consumer library
into the main directory files and DBus generated files. Those will be
addressed in follow up CLs.

Bug: 25197634
Test: FEATURES=test emerge-link update_engine; `mm` on Brillo.

Change-Id: Id8d0204ea573627e6e26ca9ea17b9592ca95bc23
diff --git a/fake_p2p_manager_configuration.h b/fake_p2p_manager_configuration.h
index 8563d6e..3d3afe4 100644
--- a/fake_p2p_manager_configuration.h
+++ b/fake_p2p_manager_configuration.h
@@ -17,17 +17,17 @@
 #ifndef UPDATE_ENGINE_FAKE_P2P_MANAGER_CONFIGURATION_H_
 #define UPDATE_ENGINE_FAKE_P2P_MANAGER_CONFIGURATION_H_
 
+#include "update_engine/common/test_utils.h"
+#include "update_engine/common/utils.h"
 #include "update_engine/p2p_manager.h"
-#include "update_engine/test_utils.h"
-#include "update_engine/utils.h"
 
 #include <string>
 #include <vector>
 
-#include <base/logging.h>
 #include <base/files/file_util.h>
-#include <base/strings/string_util.h>
+#include <base/logging.h>
 #include <base/strings/string_number_conversions.h>
+#include <base/strings/string_util.h>
 
 namespace chromeos_update_engine {