profiles: Introduce 'profile-search-paths' and use it.

* guix/profiles.scm (profile-search-paths): New procedure.
* guix/scripts/environment.scm (evaluate-search-paths): Remove.
(create-environment): Replace 'paths' with 'manifest'.  Use
'profile-search-paths' instead of 'evaluate-search-paths'.
(show-search-paths): Likewise.
(launch-environment): Replace 'paths' with 'manifest'.  Make 'pure?' a
keyword parameter.
(launch-environment/fork, launch-environment/container): Likewise.
(guix-environment): Remove 'paths' variable.  Adjust callers of the
above procedures accordingly.