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/fetch-to-store.hh

@@ -18,7 +18,7 @@ StorePath fetchToStore(
     const SourcePath & path,
     FetchMode mode,
     std::string_view name = "source",
-    ContentAddressMethod method = FileIngestionMethod::NixArchive,
+    ContentAddressMethod method = ContentAddressMethod::Raw::NixArchive,
     PathFilter * filter = nullptr,
     RepairFlag repair = NoRepair);