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

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

include/flatbuffers/util.h

@@ -448,12 +448,16 @@ bool LoadFile(const char* name, bool binary, std::string* buf);
 // If "binary" is false data is written using ifstream's
 // text mode, otherwise data is written with no
 // transcoding.
+// NOTE: this function is deprecated in favor of FileSaver class,
+// but left here for compatibility.
 bool SaveFile(const char* name, const char* buf, size_t len, bool binary);
 
 // Save data "buf" into file "name" returning true if
 // successful, false otherwise.  If "binary" is false
 // data is written using ifstream's text mode, otherwise
 // data is written with no transcoding.
+// NOTE: this function is deprecated in favor of FileSaver class,
+// but left here for compatibility.
 inline bool SaveFile(const char* name, const std::string& buf, bool binary) {
   return SaveFile(name, buf.c_str(), buf.size(), binary);
 }