Deduplicate specification of valid package names

- Rename the "Package name normalization" specification into "Package
  name grammar and normalization", to make it explicitly about the
  format of valid names too.

- Link to this spec in the core metadata specification instead of
  duplicating the part about valid names.

source/specifications/core-metadata.rst

@@ -73,15 +73,10 @@ Name
 
 .. versionadded:: 1.0
 .. versionchanged:: 2.1
-   Added additional restrictions on format from :pep:`508`
+   Added restrictions on format from the :ref:`name grammar <name-grammar>`.
 
 The name of the distribution. The name field is the primary identifier for a
-distribution. A valid name consists only of ASCII letters and numbers, period,
-underscore and hyphen. It must start and end with a letter or number.
-Distribution names are limited to those which match the following
-regex (run with ``re.IGNORECASE``)::
-
-    ^([A-Z0-9]|[A-Z0-9][A-Z0-9._-]*[A-Z0-9])$
+distribution. It must conform to the :ref:`package name grammar <name-grammar>`.
 
 Example::
 

source/specifications/name-normalization.rst

@@ -1,13 +1,16 @@
-.. _name-normalization:
+======================================
+Package name grammar and normalization
+======================================
+
+Project names obey a restricted format, and are "normalized" for use in various
+contexts. This document describes what the valid project names are, and how
+project names should be normalized.
 
-==========================
-Package name normalization
-==========================
 
-Project names are "normalized" for use in various contexts. This document describes how project names should be normalized.
+.. _name-grammar:
 
-Valid non-normalized names
---------------------------
+Name grammar
+============
 
 A valid name consists only of ASCII letters and numbers, period,
 underscore and hyphen. It must start and end with a letter or number.
@@ -16,10 +19,15 @@ following regex (run with ``re.IGNORECASE``)::
 
     ^([A-Z0-9]|[A-Z0-9][A-Z0-9._-]*[A-Z0-9])$
 
-Normalization
--------------
 
-The name should be lowercased with all runs of the characters ``.``, ``-``, or ``_`` replaced with a single ``-`` character. This can be implemented in Python with the re module:
+.. _name-normalization:
+
+Name normalization
+==================
+
+The name should be lowercased with all runs of the characters ``.``, ``-``, or
+``_`` replaced with a single ``-`` character. This can be implemented in Python
+with the re module:
 
 .. code-block:: python
 
@@ -30,7 +38,7 @@ The name should be lowercased with all runs of the characters ``.``, ``-``, or `
 
 This means that the following names are all equivalent:
 
-* ``friendly-bard``  (normalized form)
+* ``friendly-bard`` (normalized form)
 * ``Friendly-Bard``
 * ``FRIENDLY-BARD``
 * ``friendly.bard``