feat(tvix/cli): add known_paths module

This module implements types used to track the set of known paths in
the context of an evaluation.

These are used to determine the build references of a derivation.

Change-Id: I81e15ae33632784e699128916485751613b231a3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7816
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/cli/src/main.rs

@@ -1,3 +1,4 @@
+mod known_paths;
 mod nix_compat;
 mod refscan;