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/mock_file_writer.h b/mock_file_writer.h
index 0d97e33..72d6a86 100644
--- a/mock_file_writer.h
+++ b/mock_file_writer.h
@@ -18,7 +18,7 @@
 #define UPDATE_ENGINE_MOCK_FILE_WRITER_H_
 
 #include <gmock/gmock.h>
-#include "update_engine/file_writer.h"
+#include "update_engine/payload_consumer/file_writer.h"
 
 namespace chromeos_update_engine {