* guix/scripts/build.scm (transform-package-inputs): New procedure.
(%transformations): Add it.
(%transformation-options, show-transformation-options-help): Add
--with-input.
* tests/scripts-build.scm ("options->transformation, with-input"):
("options->transformation, with-input, no matches"): New tests.
* tests/guix-build.sh: Add tests.
* doc/guix.texi (Package Transformation Options): Document it.