Simplify ContentAddress

Whereas `ContentAddressWithReferences` is a sum type complex because different
varieties support different notions of reference, and
`ContentAddressMethod` is a nested enum to support that,
`ContentAddress` can be a simple pair of a method and hash.

`ContentAddress` does not need to be a sum type on the outside because
the choice of method doesn't effect what type of hashes we can use.

Co-Authored-By: Cale Gibbard <cgibbard@gmail.com>

src/libexpr/primops/fetchTree.cc

@@ -254,10 +254,8 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
         auto expectedPath = state.store->makeFixedOutputPath(
             name,
             FixedOutputInfo {
-                .hash = {
-                    .method = unpack ? FileIngestionMethod::Recursive : FileIngestionMethod::Flat,
-                    .hash = *expectedHash,
-                },
+                .method = unpack ? FileIngestionMethod::Recursive : FileIngestionMethod::Flat,
+                .hash = *expectedHash,
                 .references = {}
             });