sphinx-quickstart: Move code to 'sphinx.cmd'

We're going to move the executable's here (or at least those that part
of the core library). The 'sphinx-build' executable was already moved,
so lets do 'sphinx-quickstart' next.

To avoid breaking packages that are using this feature directly, aliases
for the old 'main' method are included. This is based on what Django
does [1] and, like Django, will allow us to safely remove the old
modules in Sphinx 2.0.

[1] https://github.com/django/django/blob/1.11/django/test/runner.py#L688-L695

Signed-off-by: Stephen Finucane <stephen@that.guru>

tests/test_quickstart.py

@@ -17,7 +17,7 @@ from six.moves import input
 import pytest
 
 from sphinx import application
-from sphinx import quickstart as qs
+from sphinx.cmd import quickstart as qs
 from sphinx.util.console import nocolor, coloron
 from sphinx.util.pycompat import execfile_