The underlying pkg library always produces a
<name>-<version>.pkg file, so to follow
CPACK_PACKAGE_FILE_NAME we need to detect that and rename appropriately.
- All supported FreeBSD versions are now using 1.17 or later, which produces
.pkgfiles. Drop support for older versions.
- Fix up some documentation where variables were listed in the wrong place as fallback.
Fixes #23034 (closed)