Add the addPassthru library function

With multiple outputs, adding attributes to a derivation without
changing the {drv,out}Path is no longer as trivial as simply using the
`//' operator, as we usually want to add the attribute to _each_ output,
and even if we only care about one that one output can be reached via
multiple paths.

For stdenv.mkDerivation, we already had code in place to add passthru
and meta attributes to derivations. This commit simply factors part of
that code out into a lib function addPassthru, which takes a derivation
and an attribute set and appends the attribute set to each output of the
derivation.

Signed-off-by: Shea Levy <shea@shealevy.com>