cmCPackGenerator: Handle CPACK_TEMPORARY_[INSTALL]_DIRECTORY properly

The variable `CPACK_TEMPORARY_DIRECTORY` is an internal variable that is
required for proper packaging. Historically, a second variable
`CPACK_TEMPORARY_INSTALL_DIRECTORY` existed as well which had the same
purpose. Both variables had to be set to the same value. Otherwise,
CPack would not succeed with packaging.
In order to ease CPack's usage, the variable
`CPACK_TEMPORARY_INSTALL_DIRECTORY` is now no longer used.

However, as it cannot be guaranteed that neither of these two (internal)
variables have been used in the wild, some mechanism was introduced to
preserve backwards-compatibility: If one of these variables is set, the
other variable will be set to the same value. If, however, both are set
to different values, CPack still fails but with an explicit
error-message.

Fixes: #25046

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