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/tvix_store_io.rs

@@ -21,6 +21,7 @@ use tokio::io::AsyncReadExt;
 use tracing::{error, instrument, warn, Level};
 use tvix_build::buildservice::BuildService;
 use tvix_eval::{ErrorKind, EvalIO, FileType, StdIO};
+use tvix_store::utils::AsyncIoBridge;
 use walkdir::DirEntry;
 
 use tvix_castore::{
@@ -341,6 +342,26 @@ impl TvixStoreIO {
         Ok(output_path)
     }
 
+    /// Transforms a BLAKE-3 digest into a SHA256 digest
+    /// by re-hashing the whole file.
+    pub(crate) async fn blob_to_sha256_hash(&self, blob_digest: B3Digest) -> io::Result<[u8; 32]> {
+        let mut reader = self
+            .blob_service
+            .open_read(&blob_digest)
+            .await?
+            .ok_or_else(|| {
+                io::Error::new(
+                    io::ErrorKind::NotFound,
+                    format!("blob represented by digest: '{}' not found", blob_digest),
+                )
+            })?;
+        // It is fine to use `AsyncIoBridge` here because hashing is not actually I/O.
+        let mut hasher = AsyncIoBridge(Sha256::new());
+
+        tokio::io::copy(&mut reader, &mut hasher).await?;
+        Ok(hasher.0.finalize().into())
+    }
+
     pub async fn store_path_exists<'a>(&'a self, store_path: StorePathRef<'a>) -> io::Result<bool> {
         Ok(self
             .path_info_service