 error messages: include whole spec for "no externals found" message (#12070)
 error messages: include whole spec for "no externals found" message (#12070)

* Provide better error message when matching externals are found for package marked not buildable

iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack