Fix #75102: `PharData` says invalid checksum for valid tar

Fix #75102: `PharData` says invalid checksum for valid tar

Apparently, there are broken tarballs out there which are actually in
ustar format, but did not write the `ustar` marker.  Since popular tar
tools like GNU tar and 7zip have no issues dealing with such tarballs,
Phar should also be more resilient.

Thus, when the first checksum check of a tarball in (presumed) in old-
style format fails, we check whether the checksum would be suitable for
ustar format; if so, we treat the tarball as being in ustar format.

Closes GH-6479.