- Spack packages were originally expected to call `from spack import *`
themselves, but it has become difficult to manage imports in the
Spack core.
- the top-level namespace polluted by package symbols, and it's not
possible to avoid circular dependencies and unnecessary module loads in
the core, given all the stuff the packages need.
- This makes the top-level `spack` package essentially empty, save for a
version tuple and a version string, and `from spack import *` is now
essentially a no-op.
- The common routines and directives that packages need are now in
`spack.pkgkit`, and the import system forces packages to automatically
include this so that old packages that call `from spack import *`
will continue to work without modification.
- Since `from spack import *` is no longer required, we could consider
removing ``from spack import *`` from packages in the future and
shifting to ``from spack.pkgkit import *``, but we can wait a while to
do this.