Provide a InputScheme::fetch() built on top of InputScheme::getAccessor()

This is for graceful migration to lazy-trees fetchers (which are all
accessor-based). Eventually fetch() will be removed.

src/libfetchers/fetchers.cc

@@ -1,5 +1,6 @@
 #include "fetchers.hh"
 #include "store-api.hh"
+#include "input-accessor.hh"
 
 #include <nlohmann/json.hpp>
 
@@ -312,6 +313,18 @@ void InputScheme::clone(const Input & input, const Path & destDir) const
     throw Error("do not know how to clone input '%s'", input.to_string());
 }
 
+std::pair<StorePath, Input> InputScheme::fetch(ref<Store> store, const Input & input)
+{
+    auto [accessor, input2] = getAccessor(store, input);
+    auto storePath = accessor->root().fetchToStore(store, input2.getName());
+    return {storePath, input2};
+}
+
+std::pair<ref<InputAccessor>, Input> InputScheme::getAccessor(ref<Store> store, const Input & input) const
+{
+    throw UnimplementedError("InputScheme must implement fetch() or getAccessor()");
+}
+
 std::optional<ExperimentalFeature> InputScheme::experimentalFeature()
 {
     return {};

src/libfetchers/fetchers.hh

@@ -9,7 +9,7 @@
 
 #include <memory>
 
-namespace nix { class Store; class StorePath; }
+namespace nix { class Store; class StorePath; struct InputAccessor; }
 
 namespace nix::fetchers {
 
@@ -148,7 +148,9 @@ struct InputScheme
         std::string_view contents,
         std::optional<std::string> commitMsg) const;
 
-    virtual std::pair<StorePath, Input> fetch(ref<Store> store, const Input & input) = 0;
+    virtual std::pair<StorePath, Input> fetch(ref<Store> store, const Input & input);
+
+    virtual std::pair<ref<InputAccessor>, Input> getAccessor(ref<Store> store, const Input & input) const;
 
     /**
      * Is this `InputScheme` part of an experimental feature?