Add tests for importing packages directly.
Add tests for importing packages directly.

Added test for, e.g.:

    import spack.pkg.builtin.mock.mpich
    import spack.pkg.builtin.mock.mpich as mpich
    from spack.pkg.builtin.mock.mpich import Mpich

Among others.  These ensure that direct package imports work so that
packages can be extended.

matthiasdiener/spack,LLNL/spack,matthiasdiener/spack,LLNL/spack,EmreAtes/spack,matthiasdiener/spack,tmerrick1/spack,LLNL/spack,LLNL/spack,skosukhin/spack,TheTimmy/spack,iulian787/spack,lgarren/spack,EmreAtes/spack,lgarren/spack,mfherbst/spack,skosukhin/spack,LLNL/spack,mfherbst/spack,EmreAtes/spack,iulian787/spack,tmerrick1/spack,skosukhin/spack,tmerrick1/spack,iulian787/spack,mfherbst/spack,skosukhin/spack,TheTimmy/spack,mfherbst/spack,krafczyk/spack,lgarren/spack,krafczyk/spack,krafczyk/spack,matthiasdiener/spack,EmreAtes/spack,krafczyk/spack,TheTimmy/spack,krafczyk/spack,iulian787/spack,iulian787/spack,mfherbst/spack,TheTimmy/spack,tmerrick1/spack,skosukhin/spack,lgarren/spack,TheTimmy/spack,lgarren/spack,EmreAtes/spack,tmerrick1/spack,matthiasdiener/spack