profiles: Manifest entries keep a reference to their parent entry.

* guix/profiles.scm (<manifest-entry>)[parent]: New field.
(package->manifest-entry): Add #:parent parameter.  Fill out the
'parent' field of <manifest-entry>; pass #:parent in recursive calls.
* guix/profiles.scm (sexp->manifest)[sexp->manifest-entry]: New
procedure.  Use it for version 3.
* tests/profiles.scm ("manifest-entry-parent"): New procedure.
("read-manifest")[entry->sexp]: Add 'manifest-entry-parent' to the
result.