emacs: Find packages in system profiles.

For a usual profile, packages are placed in a profile directory itself,
but for a system profile, packages are placed in 'profile'
sub-directory.  So we need to do some special cases for system profiles
to find packages there as well.

* emacs/guix-base.el (guix-packages-profile): New procedure.
  (guix-manifest-file): Use it.  Add optional 'system?' argument.
* emacs/guix-ui-generation.el (guix-system-generation?)
  (guix-generation-current-packages-profile):  New procedures.
  (guix-generation-packages, guix-generation-insert-packages): Remove
  'generation' argument.
  (guix-generation-packages-buffer): Add optional 'system?' argument.
  (guix-profile-generation-manifest-file)
  (guix-profile-generation-packages-buffer): Adjust accordingly.
* emacs/guix-main.scm (generation-package-specifications+paths): Rename to...
  (profile->specifications+paths): ... this.  Use a single 'profile' argument.