Add labels to source-distribution-format.rst for cross-referencing

Specifically cover source trees and sdist as files.

Author: brettcannon

source/specifications/source-distribution-format.rst

@@ -20,6 +20,8 @@ specification.
 
 Source distributions are also known as *sdists* for short.
 
+.. _source-distribution-format-source-tree:
+
 Source trees
 ============
 
@@ -30,6 +32,8 @@ directories. :pep:`517` and :pep:`518` specify what is required to meet the
 definition of what :file:`pyproject.toml` must contain for something to be
 deemed a source tree.
 
+.. _source-distribution-format-sdist:
+
 Source distribution file name
 =============================