Spack offers a handy feature allowing you to use pre-built packages already existing on the system by specifying them in packages.yaml. However, inclusion of those packages can cause pollution of paths, causing builds to pick up (for example) system packages when that was not intended. A workaround was suggested in #1794 (see the 5th comment in the discussion).

But the "right" way to solve this problem, it seems, would be to create symlinks to "extract" a single package out of the system tree. For example, I might create a tree ~/my/openssl that would symlink JUST to the openssl files from the system. That way, Spack would never be using a path like /usr/.

How to do this would vary from system to system. On my CentOS 7 system, the command to list the package is repoquery -l qt. Of course, if the package is installed in a random place, there's no systematic way to extract it. That's probably not a problem we need to worry about (randomly built packages should probably be replaced with Spack builds anyway).

I think a simple procedure should be available to do this extraction. I suspect it shouldn't be part of Spack because it's OS-specific and because it should probably be done manually for each System package you want to put in packages.yaml.