guix build: Move package transformation options behind '--help-transform'.

This change declutters the '--help' output.

* guix/scripts/build.scm (show-build-options-help)
(%standard-build-options): Add '--help-transform'.
(show-transformation-options-help): Make private.
(show-help): Remove call to 'show-transformation-options-help'.
* guix/scripts/build.scm (show-help): Likewise.
* guix/scripts/environment.scm (show-help): Likewise.
* guix/scripts/graph.scm (show-help): Likewise.
* guix/scripts/install.scm (show-help): Likewise.
* guix/scripts/pack.scm (show-help): Likewise.
* guix/scripts/package.scm (show-help): Likewise.
* guix/scripts/upgrade.scm (show-help): Likewise.
* doc/guix.texi (Package Transformation Options): Mention '--help-transform'.

doc/guix.texi

@@ -9803,6 +9803,11 @@ Package transformation options are preserved across upgrades:
 @command{guix upgrade} attempts to apply transformation options
 initially used when creating the profile to the upgraded packages.
 
+The available options are listed below.  Most commands support them and
+also support a @option{--help-transform} option that lists all the
+available options and a synopsis (these options are not shown in the
+@option{--help} output for brevity).
+
 @table @code
 
 @item --with-source=@var{source}

guix/scripts/build.scm

@@ -65,7 +65,6 @@
             %transformation-options
             options->transformation
             manifest-entry-with-transformations
-            show-transformation-options-help
 
             guix-build
             register-root
@@ -717,6 +716,8 @@ options handled by 'set-build-options-from-command-line', and listed in
   -c, --cores=N          allow the use of up to N CPU cores for the build"))
   (display (G_ "
   -M, --max-jobs=N       allow at most N build jobs"))
+  (display (G_ "
+      --help-transform   list package transformation options not shown here"))
   (display (G_ "
       --debug=LEVEL      produce debugging output at LEVEL")))
 
@@ -853,7 +854,14 @@ use '--no-offload' instead~%")))
                     (if c
                         (apply values (alist-cons 'max-jobs c result) rest)
                         (leave (G_ "not a number: '~a' option argument: ~a~%")
-                               name arg)))))))
+                               name arg)))))
+        (option '("help-transform") #f #f
+                (lambda _
+                  (format #t
+                          (G_ "Available package transformation options:~%"))
+                  (show-transformation-options-help)
+                  (newline)
+                  (exit 0)))))
 
 
 ;;;
@@ -910,8 +918,6 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
   (newline)
   (show-build-options-help)
   (newline)
-  (show-transformation-options-help)
-  (newline)
   (display (G_ "
   -h, --help             display this help and exit"))
   (display (G_ "

guix/scripts/environment.scm

@@ -179,8 +179,6 @@ COMMAND or an interactive shell in that environment.\n"))
   (newline)
   (show-build-options-help)
   (newline)
-  (show-transformation-options-help)
-  (newline)
   (display (G_ "
   -h, --help             display this help and exit"))
   (display (G_ "

guix/scripts/graph.scm

@@ -35,8 +35,7 @@
   #:use-module ((guix diagnostics)
                 #:select (location-file formatted-message))
   #:use-module ((guix scripts build)
-                #:select (show-transformation-options-help
-                          options->transformation
+                #:select (options->transformation
                           %standard-build-options
                           %transformation-options))
   #:use-module (srfi srfi-1)
@@ -546,8 +545,6 @@ Emit a representation of the dependency graph of PACKAGE...\n"))
   (display (G_ "
   -L, --load-path=DIR    prepend DIR to the package module search path"))
   (newline)
-  (show-transformation-options-help)
-  (newline)
   (display (G_ "
   -h, --help             display this help and exit"))
   (display (G_ "

guix/scripts/install.scm

@@ -38,8 +38,6 @@ This is an alias for 'guix package -i'.\n"))
   (newline)
   (show-build-options-help)
   (newline)
-  (show-transformation-options-help)
-  (newline)
   (display (G_ "
   -h, --help             display this help and exit"))
   (display (G_ "

guix/scripts/pack.scm

@@ -1043,8 +1043,6 @@ last resort for relocation."
 Create a bundle of PACKAGE.\n"))
   (show-build-options-help)
   (newline)
-  (show-transformation-options-help)
-  (newline)
   (display (G_ "
   -f, --format=FORMAT    build a pack in the given FORMAT"))
   (display (G_ "

guix/scripts/package.scm

@@ -397,8 +397,6 @@ Install, remove, or upgrade packages in a single transaction.\n"))
   (newline)
   (show-build-options-help)
   (newline)
-  (show-transformation-options-help)
-  (newline)
   (display (G_ "
   -h, --help             display this help and exit"))
   (display (G_ "

guix/scripts/upgrade.scm

@@ -41,8 +41,6 @@ This is an alias for 'guix package -u'.\n"))
   (newline)
   (show-build-options-help)
   (newline)
-  (show-transformation-options-help)
-  (newline)
   (display (G_ "
   -h, --help             display this help and exit"))
   (display (G_ "