Add public interfaces for accessing builtin package info (Bug#78844)

* lisp/emacs-lisp/package.el (package-builtin-packages)
(package-builtin-package-version): New functions.
* doc/lispref/package.texi: Add a section 'Package Information' with
documentation for the new functions.
* etc/NEWS: Add entry for 'package-versioned-builtin-packages' and
'package-builtin-package-version'.

doc/lispref/package.texi

@@ -424,3 +424,28 @@ For further details on how to make use of the package, see
 the manual installed, you can also read the
 @url{https://elpa.gnu.org/packages/doc/compat.html#Usage, Online
 Compat manual}.
+
+@node Package Information
+@section Get built-in packages and versions
+@cindex package information
+
+Emacs ships built-in packages which may also be packaged externally on
+ELPA.  Knowing whether a built-in package has a higher version than on
+ELPA or other packaging systems may be useful for external packaging
+tools.  Those who would like to query such information can use the
+following functions.
+
+@defun package-versioned-builtin-packages
+This function returns the list of packages that are built-in to Emacs
+and have a version.  The returned list contains one symbol for each
+built-in package that has a version, which are likely also packaged
+externally on ELPA.
+@end defun
+
+@defun package-builtin-package-version package
+This function returns the version of a built-in @var{package}.
+@var{package} should be a symbol.  The returned value is a list of
+integers representing the version of PACKAGE in the format returned by
+the function @code{version-to-list}, or nil if @var{package} is built-in
+but has no version or is not a built-in package.
+@end defun

etc/NEWS

@@ -2025,6 +2025,14 @@ installed, total number of packages from all the package archives, total
 number of packages to upgrade and total number of new packages
 available.
 
++++
+*** New functions to query builtin package information.
+'package-versioned-builtin-packages' returns a list of symbols of
+built-in packages; 'package-builtin-package-version' returns the
+version-list of a given package symbol.  These functions provide public
+interfaces for external tools to query information about built-in
+packages.
+
 ** Rcirc
 
 +++

lisp/emacs-lisp/package.el

@@ -669,6 +669,21 @@ called via `package-activate-all'.  To change which packages are
 loaded and/or activated, customize `package-load-list'.")
 (put 'package-alist 'risky-local-variable t)
 
+;;;; Public interfaces for accessing built-in package info
+
+(defun package-versioned-builtin-packages ()
+  "Return a list of all the versioned built-in packages.
+The return value is a list of names of built-in packages represented as
+symbols."
+  (mapcar #'car package--builtin-versions))
+
+(defun package-builtin-package-version (package)
+  "Return the version of a built-in PACKAGE given by its symbol.
+The return value is a list of integers representing the version of
+PACKAGE, in the format returned by `version-to-list', or nil if the
+package is built-in but has no version or is not a built-in package."
+  (alist-get package package--builtin-versions))
+
 ;;;###autoload
 (defvar package-activated-list nil
   ;; FIXME: This should implicitly include all builtin packages.