* guix/profiles.scm (packages->manifest)[inferiors-loaded?]: New
variable.
[inferior->entry]: New procedure.
Accept inferior packages when INFERIORS-LOADED? is true.
* tests/guix-package.sh: Add test using a manifest with an inferior.
* tests/inferior.scm ("packages->manifest"): New test.
* guix/inferior.scm (inferior-package->manifest-entry): New procedure.
* tests/inferior.scm (manifest-entry->list): New procedure.
("inferior-package->manifest-entry"): New test.
* guix/inferior.scm (<inferior>)[packages, table]: New fields.
(open-inferior): Initialize these new fields.
(inferior-packages): Rename to...
(%inferior-packages): ... this.
(inferior-packages): New procedure; force the promise.
(%inferior-package-table, lookup-inferior-packages): New procedures.
* tests/inferior.scm ("lookup-inferior-packages")
("lookup-inferior-packages and eq?-ness"): New tests.
* guix/inferior.scm (read-inferior-response)
(send-inferior-request): New procedures.
(inferior-eval): Rewrite in terms of these.
(proxy, inferior-package-derivation, inferior-package->derivation)
(package-compiler): New procedures.
* tests/inferior.scm ("inferior-package-derivation"): New test.
