Add (guix profiles).

* guix/scripts/package.scm (show-what-to-remove/install): New procedure,
  moved from...
  (guix-package): ... here.
  (<manifest>, make-manifest, <manifest-entry>,
  profile-manifest, manifest->sexp, sexp->manifest, read-manifest,
  write-manifest, remove-manifest-entry, manifest-remove,
  manifest-installed?, manifest=?, profile-regexp, generation-numbers,
  previous-generation-number, profile-derivation, generation-number,
  generation-file-name, generation-time, lower-input): Move to...
* guix/profiles.scm: ... here.  New file.
* Makefile.am (MODULES): Add it.