Seperate out the concept of the executable used by an ExternalFD
downloader and the name of the class. I'm not entirely sure why we
care aobut the name of the class at all, but it's used outside of
classes to initialize _BY_NAME() and soforth, so it seems impractical
to just change get_basename() to return the executable name
So instead, call get_execname() not get_basename().
Default get_execname() to calling get_basename(), but override it in
FFmpegFD, where it returns self.execname, which is set in
_call_downloader().
Perhaps there is a less complicated way to achieve this goal?
095cda627e