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.

lib/spack/docs/conf.py

@@ -111,28 +111,6 @@
 sphinx_apidoc(apidoc_args + ['../spack'])
 sphinx_apidoc(apidoc_args + ['../llnl'])
 
-#
-# Exclude everything in spack.__all__ from indexing.  All of these
-# symbols are imported from elsewhere in spack; their inclusion in
-# __all__ simply allows package authors to use `from spack import *`.
-# Excluding them ensures they're only documented in their "real" module.
-#
-# This also avoids issues where some of these symbols shadow core spack
-# modules.  Sphinx will complain about duplicate docs when this happens.
-#
-import fileinput
-handling_spack = False
-for line in fileinput.input('spack.rst', inplace=1):
-    if handling_spack:
-        if not line.startswith('    :noindex:'):
-            print('    :noindex: %s' % ' '.join(spack.__all__))
-        handling_spack = False
-
-    if line.startswith('.. automodule::'):
-        handling_spack = (line == '.. automodule:: spack\n')
-
-    sys.stdout.write(line)
-
 # Enable todo items
 todo_include_todos = True