refactor(tvix/castore): add try_into_anonymous_node, rename to try_*

We have two places where we parse protos and want their names to be
empty:

 - Receiving a root node in a nar-bridge NAR request
 - Processing the CalculateNAR gRPC call

We don't have any place where we want to keep a name as bytes::Bytes
around, yet we used the `into_name_bytes_and_node` method.

It was also a bit wrongly named - it wasn't very clear the name was
not validated, and that the function may fail.

This moves the "splitting off the name as bytes::Bytes" part into a
private helper, only leaving the `try_into_name_and_node` and
`try_into_anonymous_node` methods around.

Change-Id: I2c7fd9871d49ec67450d7efa6a30d96197fb319c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12664
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Marijan Petričević <marijan.petricevic94@gmail.com>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/castore/src/errors.rs

@@ -52,6 +52,10 @@ pub enum DirectoryError {
     /// Invalid name encountered
     #[error("Invalid name: {0}")]
     InvalidName(PathComponentError),
+    /// This can occur if a protobuf node with a name is passed where we expect
+    /// it to be anonymous.
+    #[error("Name is set when it shouldn't")]
+    NameInAnonymousNode,
     /// Elements are not in sorted order. Can only happen on protos
     #[error("{:?} is not sorted", .0.as_bstr())]
     WrongSorting(bytes::Bytes),