packages: 'package-mapping' correctly recurses into 'replacement'.

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.