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/libexpr/eval.cc

@@ -2303,7 +2303,7 @@ StorePath EvalState::copyPathToStore(NixStringContext & context, const SourcePat
                 path.resolveSymlinks(),
                 settings.readOnlyMode ? FetchMode::DryRun : FetchMode::Copy,
                 path.baseName(),
-                FileIngestionMethod::NixArchive,
+                ContentAddressMethod::Raw::NixArchive,
                 nullptr,
                 repair);
             allowPath(dstPath);