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/perl/lib/Nix/Store.xs

@@ -335,7 +335,7 @@ SV *
 StoreWrapper::addToStore(char * srcPath, int recursive, char * algo)
     PPCODE:
         try {
-            auto method = recursive ? FileIngestionMethod::NixArchive : FileIngestionMethod::Flat;
+            auto method = recursive ? ContentAddressMethod::Raw::NixArchive : ContentAddressMethod::Raw::Flat;
             auto path = THIS->store->addToStore(
                 std::string(baseNameOf(srcPath)),
                 PosixSourceAccessor::createAtRoot(srcPath),