Move /src to /subprojects

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

subprojects/libfetchers/store-path-accessor.cc

@@ -0,0 +1,15 @@
+#include "store-path-accessor.hh"
+#include "store-api.hh"
+
+namespace nix {
+
+ref<SourceAccessor> makeStorePathAccessor(ref<Store> store, const StorePath & storePath)
+{
+    // FIXME: should use `store->getFSAccessor()`
+    auto root = std::filesystem::path{store->toRealPath(storePath)};
+    auto accessor = makeFSSourceAccessor(root);
+    accessor->setPathDisplay(root.string());
+    return accessor;
+}
+
+}