fix(tvix/eval): getContext merges underlying values

Previously, we were assembling very naively an attribute set composed of context we saw.

But it was forgetting that `"${drv}${drv.drvPath}"` would contain 2 contexts with the same key, but
with different values, one with `outputs = [ "out" ];` and `allOutputs = true;`.

Following this reasoning and comparing with what Nix does, we ought to merge underlying values systematically.

Hence, I bring `itertools` to perform a group by on the key and merge everything on the fly, it's not
beautiful but it's the best I could find, notice that I don't use
`group_by` but I talk about group by, that is, because `group_by` is a
`group_by_consecutive`, see
rust-itertools/itertools#374.

Initially, I tried to do it without a `into_grouping_map_by`, it was akin to assemble the final `NixAttrs` directly,
it was less readable and harder to pull out because we don't have a lot of in-place mutable functions on
our data structures.

Change-Id: I9933c9bd88ffe04de50dda14f21879b60d8b8cd4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10620
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>