refactor(tvix/glue): move Fetch[er] into its own types, fetch lazily

We actually want to delay fetching until we actually need the file. A
simple evaluation asking for `.outPath` or `.drvPath` should work even
in a pure offline environment.

Before this CL, the fetching logic was quite distributed between
tvix_store_io, and builtins/fetchers.rs.

Rather than having various functions and conversions between structs,
describe a Fetch as an enum type, with the fields describing the fetch.

Define a store_path() function on top of `Fetch` which can be used to
ask for the calculated store path (if the digest has been provided
upfront).

Have a `Fetcher` struct, and give it a `fetch_and_persist` function,
taking a `Fetch` as well as a desired name, and have it deal with all
the logic of persisting the PathInfos. It also returns a StorePathRef,
similar to the `.store_path()` method on a `Fetch` struct.

In a followup CL, we can extend KnownPaths to track fetches AND
derivations, and then use `Fetcher` when we need to do IO into that
store path.

Change-Id: Ib39a96baeb661750a8706b461f8ba4abb342e777
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11500
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/glue/src/builtins/errors.rs

@@ -41,17 +41,17 @@ pub enum FetcherError {
     #[error("Invalid hash type '{0}' for fetcher")]
     InvalidHashType(&'static str),
 
-    #[error("Error in store path for fetcher output: {0}")]
-    StorePath(#[from] BuildStorePathError),
-
     #[error(transparent)]
     Http(#[from] reqwest::Error),
-}
 
-impl From<FetcherError> for tvix_eval::ErrorKind {
-    fn from(err: FetcherError) -> Self {
-        tvix_eval::ErrorKind::TvixError(Rc::new(err))
-    }
+    #[error(transparent)]
+    Io(#[from] std::io::Error),
+
+    #[error(transparent)]
+    Import(#[from] tvix_castore::import::Error),
+
+    #[error("Error calculating store path for fetcher output: {0}")]
+    StorePath(#[from] BuildStorePathError),
 }
 
 /// Errors related to `builtins.path` and `builtins.filterSource`,