init: move symbols in `spack` to `spack.pkgkit`

- 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.

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