packages.yaml: allow virtuals to specify buildable: false (#14934)

Currently, to force Spack to use an external MPI, you have to specify `buildable: False`
for every MPI provider in Spack in your packages.yaml file. This is both tedious and
fragile, as new MPI providers can be added and break your workflow when you do a
git pull.

This PR allows you to specify an entire virtual dependency as non-buildable, and
specify particular implementations to be built:

```
packages:
all:
    providers:
        mpi: [mpich]
mpi:
    buildable: false
    paths:
        mpich@3.2 %gcc@7.3.0: /usr/packages/mpich-3.2-gcc-7.3.0
```
will force all Spack builds to use the specified `mpich` install.