refactor(tvix/derivation): use BTreeSet for derivation outputs

When constructing derivations inside builtins.derivationStrict, we'd
have to very frequently check whether certain outputs have already
been inserted into the derivation inputs.

Using a set type is much easier, especially as this has to be ordered
and the source data that is being inserted also comes from a set,
which might let us pass this more efficiently in the future.

Note that the validate function no longer checks the order of the
entries, as that is now guaranteed by the type.

Change-Id: I2fbb984facba3e668075f6f8df8992092368c63d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7826
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/derivation/src/write.rs

@@ -1,5 +1,6 @@
 use crate::output::Output;
 use crate::string_escape::escape_string;
+use std::collections::BTreeSet;
 use std::{collections::BTreeMap, fmt, fmt::Write};
 
 pub const DERIVATION_PREFIX: &str = "Derive";
@@ -84,7 +85,7 @@ pub fn write_outputs(
 
 pub fn write_input_derivations(
     writer: &mut impl Write,
-    input_derivations: &BTreeMap<String, Vec<String>>,
+    input_derivations: &BTreeMap<String, BTreeSet<String>>,
 ) -> Result<(), fmt::Error> {
     writer.write_char(COMMA)?;
     writer.write_char(BRACKET_OPEN)?;