Provide a self-contained PDI distribution using spack

Subtask of #147 (closed).

Once spack recipes have been written for PDI and its dependencies, we should provide a self-contained PDI distribution tarball using it:

1. include the spack executable so that the user doesn't have to retrieve it from elsewhere
2. extract and include the sub-part of the spack recipes that are useful for PDI,
3. modify the recipes with local URL in the tarball for sources so as not to rely on internet access,
4. use cmake to generate a spack config that points to preinstalled libraries so as to only recompile what's required.