Git fetcher: Improve submodule handling

Instead of making a complete copy of the repo, fetching the
submodules, and writing the result to the store (which is all
superexpensive), we now fetch the submodules recursively using the Git
fetcher, and return a union accessor that "mounts" the accessors for
the submodules on top of the root accessor.

src/libfetchers/union-input-accessor.hh

@@ -0,0 +1,9 @@
+#pragma once
+
+#include "input-accessor.hh"
+
+namespace nix {
+
+ref<InputAccessor> makeUnionInputAccessor(std::map<CanonPath, ref<InputAccessor>> mounts);
+
+}