(isdir): Remove decl.

(isdir): Remove decl.
(install_file_to_path): Rely on make_path to fail if the destination
is not a directory, by passing preserve_existing==true to it.
Hence we no longer need to call isdir.
Free dest_dir immediately when it's no longer needed, rather than
waiting until the end of the function.
(copy_file): Don't bother calling isdir, as copy will do the
right thing if the destination is a directory.