Rename fpdfsdk/fpdfxfa files to match contents

Each of these files contains a single class, rename the file to match the
internal class name.

Review-Url: https://codereview.chromium.org/2385423004
diff --git a/fpdfsdk/cpdfsdk_annothandlermgr.cpp b/fpdfsdk/cpdfsdk_annothandlermgr.cpp
index 0fac080..32ff8d6 100644
--- a/fpdfsdk/cpdfsdk_annothandlermgr.cpp
+++ b/fpdfsdk/cpdfsdk_annothandlermgr.cpp
@@ -19,7 +19,7 @@
 
 #ifdef PDF_ENABLE_XFA
 #include "fpdfsdk/cpdfsdk_xfawidgethandler.h"
-#include "fpdfsdk/fpdfxfa/fpdfxfa_page.h"
+#include "fpdfsdk/fpdfxfa/cpdfxfa_page.h"
 #include "xfa/fxfa/xfa_ffpageview.h"
 #include "xfa/fxfa/xfa_ffwidget.h"
 #endif  // PDF_ENABLE_XFA