CMakePackageConfigHelpers: Restore undocumented PACKAGE_PREFIX_DIR variable

Revert commit 6ddf8712cd (CMakePackageConfigHelpers: Use unique variable
name for pkg prefix, 2024-03-30, v3.29.1~8^2).  It regressed existing
projects that rely on the undocumented `PACKAGE_PREFIX_DIR` variable in
package configuration files generated by `CMakePackageConfigHelpers`.

Fixes: #25873
Fixes: #25885
Issue: #25827
6 files changed