feat(tvix/eval): implement builtins.path

Now, it supports almost everything except `recursive = false;`, i.e. `flat`-ingestion
because we have no knob exposed in the tvix store import side to do it.

This has been tested to work.

Change-Id: I2e9da10ceccdfbf45b43c532077ed45d6306aa98
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10597
Tested-by: BuildkiteCI
Autosubmit: raitobezarius <tvl@lahfa.xyz>
Reviewed-by: flokli <flokli@flokli.de>

tvix/glue/src/builtins/errors.rs

@@ -53,3 +53,19 @@ impl From<FetcherError> for tvix_eval::ErrorKind {
         tvix_eval::ErrorKind::TvixError(Rc::new(err))
     }
 }
+
+/// Errors related to `builtins.path` and `builtins.filterSource`,
+/// a.k.a. "importing" builtins.
+#[derive(Debug, Error)]
+pub enum ImportError {
+    #[error("non-file '{0}' cannot be imported in 'flat' mode")]
+    FlatImportOfNonFile(String),
+    #[error("hash mismatch at ingestion of '{0}', expected: '{1}', got: '{2}'")]
+    HashMismatch(String, NixHash, NixHash),
+}
+
+impl From<ImportError> for tvix_eval::ErrorKind {
+    fn from(err: ImportError) -> Self {
+        tvix_eval::ErrorKind::TvixError(Rc::new(err))
+    }
+}