Implement --file-names-only (#8788)

* flatc builds and seems to work, some of the extra targets are having linker errors

* fix build system

* pipeline failures

* un-rename files

* refactor to use unique_ptr

* typo

* rm make_unique, add comments

* fix cmake

---------

Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>

src/idl_gen_php.cpp

@@ -95,7 +95,7 @@ class PhpGenerator : public BaseGenerator {
 
     std::string filename =
         NamespaceDir(*def.defined_namespace) + def.name + ".php";
-    return SaveFile(filename.c_str(), code, false);
+    return parser_.opts.file_saver->SaveFile(filename.c_str(), code, false);
   }
 
   // Begin a class declaration.