feat(tvix/build): add derivation_to_build_request

This function converts from a nix_compat::derivation::Derivation to
a BuildRequest.

In addition to the Derivation itself, it needs two lookup functions to
map input paths to their castore nodes.

Change-Id: I0332982f0bc7933a5fda137fe39d5a850639d929
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10236
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/glue/src/lib.rs

@@ -1,6 +1,7 @@
 pub mod builtins;
 pub mod known_paths;
 pub mod refscan;
+pub mod tvix_build;
 pub mod tvix_io;
 pub mod tvix_store_io;