Steps to reproduce

1. Spec an external installation of MKL as intel-oneapi-mkl +cluster in packages.yaml
2. Install any package with the above intel-oneapi-mkl +cluster as a dependency.

This bug was introduced in #37386. Since externals don't have dependencies in their specs, the reference to mpi fails and results in an error.

It would be ideal to have a fix which accomplishes the same goal as the above PR but in a way which works for external MKL as well, as the PR is valuable for fixing build errors which result when building serial libraries, for example blaspp ^intel-oneapi-mkl +cluster.

Error message

==> Error: KeyError: 'No spec with name mpi in intel-oneapi-mkl@2023.1.0%oneapi@2023.1.0+cluster~envmods~ilp64+shared build_system=generic arch=linux-sles15-x86_64'

.../spack/var/spack/repos/builtin/packages/intel-oneapi-mkl/package.py:168, in _find_mkl_libs:
        165
        166        resolved_libs = find_libraries(libs, lib_path, shared=shared)
        167        if "+cluster" in self.spec:
  >>    168            resolved_libs = resolved_libs + self.spec["mpi"].libs
        169        return resolved_libs

Information on your system

• Spack: 0.20.0.dev0 (b1b7dab180ba0be240dd8983d8173fa1590ef080)
• Python: 3.6.15
• Platform: linux-sles15-icelake
• 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