PEP 517: Add sdist and build preparation hooks

Bring PEP 517 up to date with latest distutils-sig discussion:

- sdist export hook
- build preparation hook for out-of-tree builds

pep-0517.txt

@@ -220,9 +220,45 @@ including any unrecognized files it created.
 
 Mandatory.
 
+::
+
+  def build_sdist(sdist_directory, config_settings):
+      ...
+
+Must build an unpacked source distribution in the specified
+``sdist_directory``.
+
+An unpacked source distribution (sdist) consists of a directory called
+``{name}-{version}`` (e.g. ``foo-1.0``), containing the source files of the
+package. This directory must also contain the
+``pyproject.toml`` from the build directory, and a PKG-INFO file containing
+metadata in the format described in
+`PEP 345 <https://www.python.org/dev/peps/pep-0345/>`_.
+
+Mandatory, but it may not succeed in all situations: for instance, some tools
+can only build an sdist from a VCS checkout.
+
+::
+
+  def prepare_build_files(build_directory, config_settings):
+      ...
+
+Must copy or create any files needed to build a wheel of this package into
+``build_directory``. For instance, it is expected  that ``pyproject.toml`` will
+exist in the root of this directory after this hook runs. For tools such as
+`setuptools_scm <https://github.com/pypa/setuptools_scm>`_, this may include
+extracting some information from a version control system.
+The ``build_wheel`` hook will subsequently be run from the ``build_directory``
+populated by this hook.
+
+Optional. If this hook is not defined, frontends may call ``build_sdist`` and
+extract the resulting tarball to use as a build directory. Backends in which
+building an sdist has additional requirements should define
+``prepare_build_files``.
+
 .. note:: Editable installs
 
-   This PEP originally specified a fourth hook, ``install_editable``, to do an
+   This PEP originally specified another hook, ``install_editable``, to do an
    editable install (as with ``pip install -e``). It was removed due to the
    complexity of the topic, but may be specified in a later PEP.