stdenv.mkDerivation: remove attrs to other outputs in multi-out deriv…

…ation sub-output attrs

In other words, previously a package 'foo' with outputs = [ "dev "bin" ]
would behave like this:

    foo.dev.bin == foo.bin
    foo.bin.dev == foo.dev == foo

After this commit this is no longer the case:

    foo.dev.bin => error: attribute ‘bin’ missing
    foo.bin.dev => error: attribute ‘dev’ missing

The rationale for this is that otherwise it's not possible to do things like:

    environment.systemPackages = [ pkgs.shadow.su ];

... where the 'shadow' package has outputs = [ "out" "su" ] and
systemPackages doing the 'pkg.bin or pkg.out or pkg' dance when choosing
which output(s) to include in the system path.

lib/customisation.nix

@@ -123,13 +123,13 @@ rec {
 
       outputToAttrListElement = outputName:
         { name = outputName;
-          value = commonAttrs // {
+          value = (builtins.removeAttrs commonAttrs outputs) // {
             inherit (drv.${outputName}) outPath drvPath type outputName;
           };
         };
 
       outputsList = map outputToAttrListElement outputs;
-  in commonAttrs.${drv.outputName};
+  in commonAttrs;
 
 
   /* Strip a derivation of all non-essential attributes, returning