sphinx-build: Move code out of 'sphinx.__init__'

We have multiple executables in tree and, while 'sphinx-build' is
arguably the most important of these, there's no reason its importance
should warrant inclusion at the package level.

Create a new module, 'sphinx.cmd', and move the code from
'sphinx.__init__' into a 'build' submodule within. This name might be a
bit disingenuous at present, given the availability of 'make-mode' here
too, but that's an artifact of the current executable design and can be
cleaned up later.

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>

setup.py

@@ -239,7 +239,7 @@ setup(
     include_package_data=True,
     entry_points={
         'console_scripts': [
-            'sphinx-build = sphinx:main',
+            'sphinx-build = sphinx.cmd.build:main',
             'sphinx-quickstart = sphinx.quickstart:main',
             'sphinx-apidoc = sphinx.ext.apidoc:main',
             'sphinx-autogen = sphinx.ext.autosummary.generate:main',