Fix an issue with dune virtual library implementations

The way dune compiles implementations of virtual libraries means that there
isn't a digest of the interface. We only use this for a convenient unique
key, so this commit adds a fallback to using the source hash intead, and
if _that_ doesn't exist, just use the name.

Author: jonludlam

src/loader/odoc_loader.ml

@@ -136,7 +136,11 @@ let read_cmt ~make_root ~parent ~filename () =
       in
       let interface = cmt_info.cmt_interface_digest in
       (match cmt_info.cmt_interface_digest with
-      | None -> raise Corrupted
+      | None ->
+        (match cmt_info.cmt_source_digest with
+        | Some x -> (try Odoc_model.Names.set_unique_ident (Digest.to_hex x) with _ -> ())
+        | None ->
+          (try Odoc_model.Names.set_unique_ident name with _ -> ()))
       | Some digest -> (
           try Odoc_model.Names.set_unique_ident (Digest.to_hex digest)
           with _ -> ()));