feat(tvix/derivation): implement Derivation::validate()

Change-Id: I87dfadda872439e108e5f678a5da63dd5b1915d1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7732
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/derivation/src/output.rs

@@ -1,4 +1,5 @@
 use serde::{Deserialize, Serialize};
+use tvix_store::nixpath::NixPath;
 
 #[derive(Serialize, Deserialize)]
 pub struct Output {
@@ -20,4 +21,9 @@ impl Output {
     pub fn is_fixed(&self) -> bool {
         self.hash.is_some()
     }
+
+    pub fn validate(&self) -> anyhow::Result<()> {
+        NixPath::from_absolute_path(&self.path)?;
+        Ok(())
+    }
 }