bug#20255: 'search-paths' should respect both user and system profile.

[宋文武]

Currently, search-paths built only from packages in user's profile.
As reported by Andy Wingo in #guix, when I have:
  perl installed into system profile
  perl-xml-parser installed into user profile
  
guix package --search-paths won't give a hint about PERL5LIB,
so it's very likely end up with a broken XML::Parser.
Another interesting fact is that we have both guile and guix in
system profile, but the guix modules isn't work out-of-the-box
on GuixSD.