guix package: Move profile cleaning out of 'search-path-environment-variables'.

* guix/scripts/package.scm (user-friendly-profile): New procedure.
  (search-path-environment-variables): Remove 'profile' local variable.
  (display-search-paths): Explicitly call 'user-friendly-profile' for
  the argument to 'search-path-environment-variables'.
  (guix-package)[process-query]: Likewise.