Let's not have a package named "lib"

See https://github.com/NixOS/nixpkgs/commit/fb24c7c95b586678c2b65e6751a7797844c44e28.

The "_" suffix follows the convention of the "if" package.

nixos/lib/testing.nix

···
 
         wrapProgram $out/bin/nixos-test-driver \
           --prefix PATH : "${qemu_kvm}/bin:${vde2}/bin:${netpbm}/bin:${coreutils}/bin" \
-          --prefix PERL5LIB : "${with perlPackages; pkgs.lib.makePerlPath [ TermReadLineGnu XMLWriter IOTty FileSlurp ]}:$out/lib/perl5/site_perl"
+          --prefix PERL5LIB : "${with perlPackages; lib.makePerlPath [ TermReadLineGnu XMLWriter IOTty FileSlurp ]}:$out/lib/perl5/site_perl"
       '';
   };
 

pkgs/top-level/perl-packages.nix

···
     };
   };
 
-  lib = buildPerlPackage {
+  lib_ = buildPerlPackage {
     name = "lib-0.63";
     src = fetchurl {
       url = mirror://cpan/authors/id/S/SM/SMUELLER/lib-0.63.tar.gz;