Cleanup ContentAddressMethod to match docs

The old `std::variant` is bad because we aren't adding a new case to
`FileIngestionMethod` so much as we are defining a separate concept ---
store object content addressing rather than file system object content
addressing. As such, it is more correct to just create a fresh
enumeration.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libfetchers/mercurial.cc

@@ -213,7 +213,7 @@ struct MercurialInputScheme : InputScheme
                 auto storePath = store->addToStore(
                     input.getName(),
                     {getFSSourceAccessor(), CanonPath(actualPath)},
-                    FileIngestionMethod::NixArchive, HashAlgorithm::SHA256, {},
+                    ContentAddressMethod::Raw::NixArchive, HashAlgorithm::SHA256, {},
                     filter);
 
                 return storePath;