Steps to reproduce

1. Choose a package with multi-valued variants (MVV)
2. Add it as an external package but with only some of those possible values enabled
3. Try to build something that requires some of the unspecified values
4. Notice that the external package satisfies those conditions despite not having those values enabled

Error message

No errors, but the concretizer shouldn't be using features undeclared in the external package. For non-MVV, we could explicitly add ~foo~bar, but there's no way to do that with MVVs.

Information on your system

• Spack: 0.19.0.dev0 (98860c6)
• Python: 3.9.12
• Platform: darwin-monterey-m1
• Concretizer: clingo

General information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have searched the issues of this repo and believe this is not a duplicate
• I have run the failing commands in debug mode and reported the output