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/fpdftext.cpp b/fpdfsdk/fpdftext.cpp
index 4244a2d..4282350 100644
--- a/fpdfsdk/fpdftext.cpp
+++ b/fpdfsdk/fpdftext.cpp
@@ -16,8 +16,8 @@
 #include "third_party/base/stl_util.h"
 
 #ifdef PDF_ENABLE_XFA
-#include "fpdfsdk/fpdfxfa/fpdfxfa_doc.h"
-#include "fpdfsdk/fpdfxfa/fpdfxfa_page.h"
+#include "fpdfsdk/fpdfxfa/cpdfxfa_document.h"
+#include "fpdfsdk/fpdfxfa/cpdfxfa_page.h"
 #endif  // PDF_ENABLE_XFA
 
 #ifdef _WIN32