install: Normalize DESTINATION paths

The file generated by install(EXPORT) computes _IMPORT_PREFIX
in a way that assumes a normalized path. If the DESTINATION
contains any ../ components, the computed _IMPORT_PREFIX
would be wrong. Force the DESTINATION path to be normalized,
subject to the new CMP0176 policy.

Also normalize all other DESTINATION paths for consistency,
except for INCLUDES DESTINATION, which is not strictly a
destination but rather a search path to add.

Fixes: #26252
22 files changed