Migrate SKIPIF -> EXTENSIONS (#7138)

This is an automated migration of most SKIPIF extension_loaded checks.

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. Sinc

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.

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