Missing header files in spack installed v2.4.2

I've installed v2.4.2 using spack v0.23.1 and our application includes `CLI/App.hpp`.  It seems that the headers in the `imp` directory are not included in the installation procedure and are not present in our installation directory.

Build log selection:

```
In file included from /home/foo/builds/XpY_9Zdj/5/bar/my_app/packages/Utilities/core/tests/tstCLI11.cc:10:
In file included from /projects/mp_common/sw_env/dev/spack/opt/spack/__spack_path_placeholder__/__spack_path_placeholder__/__spack_path_placeholder___/linux-rhel8-zen2/gcc-14.2.0/cli11-2.4.2-shhrifkzyllkcdbwiynww7igmvto57ah/share/cmake/CLI11/../../../include/CLI/App.hpp:27:
In file included from /projects/mp_common/sw_env/dev/spack/opt/spack/__spack_path_placeholder__/__spack_path_placeholder__/__spack_path_placeholder___/linux-rhel8-zen2/gcc-14.2.0/cli11-2.4.2-shhrifkzyllkcdbwiynww7igmvto57ah/share/cmake/CLI11/../../../include/CLI/ConfigFwd.hpp:20:
In file included from /projects/mp_common/sw_env/dev/spack/opt/spack/__spack_path_placeholder__/__spack_path_placeholder__/__spack_path_placeholder___/linux-rhel8-zen2/gcc-14.2.0/cli11-2.4.2-shhrifkzyllkcdbwiynww7igmvto57ah/share/cmake/CLI11/../../../include/CLI/Error.hpp:21:
/projects/mp_common/sw_env/dev/spack/opt/spack/__spack_path_placeholder__/__spack_path_placeholder__/__spack_path_placeholder___/linux-rhel8-zen2/gcc-14.2.0/cli11-2.4.2-shhrifkzyllkcdbwiynww7igmvto57ah/share/cmake/CLI11/../../../include/CLI/StringTools.hpp:268:10: fatal error: 'impl/StringTools_inl.hpp' file not found
  268 | #include "impl/StringTools_inl.hpp"  // IWYU pragma: export
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~
```

From the app source file, `tstCLI11.cc`, starting at line 10:
```
#include "CLI/App.hpp"
#include "CLI/Config.hpp"
#include "CLI/Formatter.hpp"
```

Our installation include directory contents:
```
$ ls -al  /projects/mp_common/sw_env/dev/spack/opt/spack/__spack_path_placeholder__/__spack_path_placeholder__/__spack_path_placeholder___/linux-rhel8-zen2/gcc-14.2.0/cli11-2.4.2-shhrifkzyllkcdbwiynww7igmvto57ah/include/CLI/
total 284
drwxr-sr-x 1 i24 users     0 Aug 14 14:33 .
drwxr-sr-x 1 i24 users     0 Aug 14 14:33 ..
-rw-r--r-- 1 i24 users 58457 May  4  2024 App.hpp
-rw-r--r-- 1 i24 users   749 May  4  2024 Argv.hpp
-rw-r--r-- 1 i24 users   720 May  4  2024 CLI.hpp
-rw-r--r-- 1 i24 users  5917 May  4  2024 ConfigFwd.hpp
-rw-r--r-- 1 i24 users  1743 May  4  2024 Config.hpp
-rw-r--r-- 1 i24 users  1702 May  4  2024 Encoding.hpp
-rw-r--r-- 1 i24 users 16022 May  4  2024 Error.hpp
-rw-r--r-- 1 i24 users  6131 May  4  2024 FormatterFwd.hpp
-rw-r--r-- 1 i24 users   632 May  4  2024 Formatter.hpp
-rw-r--r-- 1 i24 users  3889 May  4  2024 Macros.hpp
-rw-r--r-- 1 i24 users 29545 May  4  2024 Option.hpp
-rw-r--r-- 1 i24 users  1764 May  4  2024 Split.hpp
-rw-r--r-- 1 i24 users 10617 May  4  2024 StringTools.hpp
-rw-r--r-- 1 i24 users  4995 May  4  2024 Timer.hpp
-rw-r--r-- 1 i24 users 71706 May  4  2024 TypeTools.hpp
-rw-r--r-- 1 i24 users 36659 May  4  2024 Validators.hpp
-rw-r--r-- 1 i24 users   458 May  4  2024 Version.hpp
```

Not sure if this is an issue on the spack installation script or in the cli11 cmake scripts.  I do see that the `CLI11_impl_headers` is included in the Cmake target `CLI11` so I don't think that's the issue, but I can't tell.  Any direction here?

[]