profiles: Represent propagated inputs as manifest entries.

profiles: Represent propagated inputs as manifest entries.

* guix/profiles.scm (package->manifest-entry): Turn DEPS into a list of
manifest entries.
(manifest->gexp)[entry->gexp]: Call 'entry->gexp' on DEPS.
Bump version to 3.
(sexp->manifest)[infer-dependency]: New procedure.
Use it for versions 1 and 2.  Parse version 3.
(manifest-inputs)[entry->gexp]: New procedure.
Adjust to 'dependencies' being a list of <manifest-entry>.
* tests/profiles.scm ("packages->manifest, propagated inputs")
("read-manifest"): New fields.