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

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'.