shill: cellular: Move cellular code to its own sub-directory.

This CL is part of a series of CLs, which aim to reorganzie shill's flat
source code structure into a more modular form by moving technology
specific code into their own sub-directories.

BUG=chromium:433419
TEST=`USE='cellular' FEATURES=test emerge-$BOARD shill`
TEST=`USE='cellular clang asan' FEATURES=test emerge-$BOARD shill`

Change-Id: I783e85d8c606426ce2ded093588c1243fd0eef97
Reviewed-on: https://chromium-review.googlesource.com/229799
Reviewed-by: Thieu Le <thieule@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
diff --git a/manager.h b/manager.h
index d0ddd65..49a506f 100644
--- a/manager.h
+++ b/manager.h
@@ -18,6 +18,7 @@
 #include <chromeos/dbus/service_constants.h>
 #include <gtest/gtest_prod.h>  // for FRIEND_TEST
 
+#include "shill/cellular/modem_info.h"
 #include "shill/crypto_util_proxy.h"
 #include "shill/device.h"
 #include "shill/device_info.h"
@@ -25,7 +26,6 @@
 #include "shill/geolocation_info.h"
 #include "shill/hook_table.h"
 #include "shill/metrics.h"
-#include "shill/modem_info.h"
 #include "shill/power_manager.h"
 #include "shill/profile.h"
 #include "shill/property_store.h"