Re: Unstable broken due to missing 'weigh' dependency

[Chris Warburton]

The way release.nix was removing unwanted values was wrong. We used a
library functions to traverse attrsets recursively, calling a given
function on each value.

Unfortunately, that function doesn't consider other attrsets to be
values: instead, it just recurses into them without giving us a chance
to mark them as unwanted.

This meant that, despite us trying to discard defs.haskell-te.haskell-te
(and everything contained within, that we haven't explicitly pulled out)
it was actually all still there, and hence why Hydra was trying to
evaluate ML4HSFE and hitting this problem.