The problem is the package_version_file name was not valid!
This works better:
bash-5.2$ !tree
tree stagedir/lib/
stagedir/lib/
├── cmake
│ └── beman.exemplar
│ ├── beman.exemplar-config-version.cmake
│ ├── beman.exemplar-config.cmake
│ ├── beman.exemplar-targets-relwithdebinfo.cmake
│ └── beman.exemplar-targets.cmake
└── libbeman.exemplar.a
3 directories, 5 files
bash-5.2$
IMHO: The possible valid names are not so easy to find in the documentation as it should!