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/libstore/make-content-addressed.cc

@@ -52,10 +52,8 @@ std::map<StorePath, StorePath> makeContentAddressed(
             dstStore,
             path.name(),
             FixedOutputInfo {
-                .hash = {
-                    .method = FileIngestionMethod::Recursive,
-                    .hash = narModuloHash,
-                },
+                .method = FileIngestionMethod::Recursive,
+                .hash = narModuloHash,
                 .references = std::move(refs),
             },
             Hash::dummy,