Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libfetchers/fetch-to-store.hh

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "source-path.hh"
+#include "store-api.hh"
+#include "file-system.hh"
+#include "repair-flag.hh"
+#include "file-content-address.hh"
+
+namespace nix {
+
+enum struct FetchMode { DryRun, Copy };
+
+/**
+ * Copy the `path` to the Nix store.
+ */
+StorePath fetchToStore(
+    Store & store,
+    const SourcePath & path,
+    FetchMode mode,
+    std::string_view name = "source",
+    ContentAddressMethod method = ContentAddressMethod::Raw::NixArchive,
+    PathFilter * filter = nullptr,
+    RepairFlag repair = NoRepair);
+
+}