mkDerivation: Allow direct access to passthru

Before this, the passthru attributes were only merged in with the
derivation attribute set, and there was no way to distinguish after the
fact which attributes were part of the derivation and which came from
passthru. Now passthru can be looked at separately as well.

Signed-off-by: Shea Levy <shea@shealevy.com>
Revert "Revert "overrideDerivation: Simplify""

Now that `drv.passthru' actually is meaningful, this works.

This reverts commit d21bfddc57cdbc6c9a878dc364ffefbfc4455488.

Signed-off-by: Shea Levy <shea@shealevy.com>
overrideDerivation: Re-attatch passthru and meta to all outputs

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