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/omaha_request_action.h b/omaha_request_action.h
index 57ef250..36d233c 100644
--- a/omaha_request_action.h
+++ b/omaha_request_action.h
@@ -28,8 +28,8 @@
 #include <brillo/secure_blob.h>
 #include <curl/curl.h>
 
-#include "update_engine/action.h"
-#include "update_engine/http_fetcher.h"
+#include "update_engine/common/action.h"
+#include "update_engine/common/http_fetcher.h"
 #include "update_engine/omaha_response.h"
 
 // The Omaha Request action makes a request to Omaha and can output