feat(tvix/eval): implement unsafeDiscardOutputDependency

This builtin only transforms any `NixContextElement::Derivation` into the trivial `NixContextElement::Plain`.

This is a forgetful functor on derivation-deep context strings.

The test coverage of this change is done in cl/11264.

Change-Id: Icd00778c97766be6db8a6bdabaa59e9724353ec5
Signed-off-by: Ryan Lahfa <tvl@lahfa.xyz>
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11262
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/value/string.rs

@@ -48,6 +48,12 @@ impl From<NixContextElement> for NixContext {
     }
 }
 
+impl From<HashSet<NixContextElement>> for NixContext {
+    fn from(value: HashSet<NixContextElement>) -> Self {
+        Self(value)
+    }
+}
+
 impl NixContext {
     /// Creates an empty context that can be populated
     /// and passed to form a contextful [NixString], albeit