The '--with-source' option is the first one that was implemented, and
it's the only one that would operate only on leaf packages rather than
traversing the dependency graph. This change makes it consistent with
the rest of the transformation options.
* guix/transformations.scm (evaluate-source-replacement-specs): New
procedure.
(transform-package-source): Rewrite using it.
* tests/transformations.scm ("options->transformation, with-source, no
matches"): Rewrite since we no longer get a warning.
("options->transformation, with-source, in depth"): New test.
* doc/guix.texi (Package Transformation Options): Adjust examples.
