Add labels to dependency-specifiers.rst for cross-referencing

Author: brettcannon

source/specifications/dependency-specifiers.rst

@@ -128,6 +128,8 @@ Whitespace
 Non line-breaking whitespace is mostly optional with no semantic meaning. The
 sole exception is detecting the end of a URL requirement.
 
+.. _dependency-specifiers-names:
+
 Names
 -----
 
@@ -142,6 +144,8 @@ with re.IGNORECASE) is::
 
     ^([A-Z0-9]|[A-Z0-9][A-Z0-9._-]*[A-Z0-9])$
 
+.. _dependency-specifiers-extras:
+
 Extras
 ------
 
@@ -159,6 +163,8 @@ are listed in the "security" extra of requests.
 
 If multiple extras are listed, all the dependencies are unioned together.
 
+.. _dependency-specifiers-versions:
+
 Versions
 --------
 
@@ -170,6 +176,8 @@ via a URL. Version comparison are also used in the markers feature. The
 optional brackets around a version are present for compatibility with
 :pep:`345` but should not be generated, only accepted.
 
+.. _dependency-specifiers-environment-markers:
+
 Environment Markers
 -------------------
 
@@ -294,6 +302,8 @@ The ``implementation_version`` marker variable is derived from
 This environment markers section, initially defined through :pep:`508`, supersedes the environment markers
 section in :pep:`345`.
 
+.. _dependency-specifiers-grammar:
+
 Complete Grammar
 ================