Suggested by Alex Kost <alezost@gmail.com>.
* guix/scripts/package.scm (options->installable)[package->manifest-entry]:
Move to (guix profiles).
[package->manifest-entry*]: New procedure.
Use it.
* guix/profiles.scm (package->manifest-entry): New procedure.
* tests/profiles.scm (guile-for-build): New variable.
Call '%guile-for-build'.
("profile-derivation"): New test.