Previously, something like:
guix build glib --with-graft=glibc=glibc@2.29
would produce a result showing that rewriting rules were not applied to
libx11@1.6.A (a replacement).
* guix/packages.scm (package-mapping): Call REPLACE instead of PROC to
'replacement'.
* tests/packages.scm ("package-input-rewriting/spec, graft"): New test.