transformations: Add '--with-latest'.

* guix/upstream.scm (upstream-source-compiler): New procedure.
(%updaters): Set! it.
* guix/transformations.scm (transform-package-latest): New procedure.
(%transformations): Add 'with-latest'.
(%transformation-options, show-transformation-options-help/detailed):
Add '--with-latest'.
* tests/transformations.scm ("options->transformation, with-latest"):
New test.
* doc/guix.texi (Package Transformation Options): Document it.