Add back copyClosure for plain StorePaths

This was removed in 2e199673a5 when
`copyPath` transitioned to use `RealisedPath`. But then in
e9848beca7 we added it back just for
`realisedPath`.

I think it is a good utility function --- one can easily imagine it
becoming optimized in the future, and copying paths *violating* the
closure is a very niche feature.

So if we have `copyPaths` for both sorts of paths, I think we should
have `copyClosure` for both sorts too.

src/libstore/store-api.hh

@@ -794,6 +794,13 @@ void copyClosure(
     CheckSigsFlag checkSigs = CheckSigs,
     SubstituteFlag substitute = NoSubstitute);
 
+void copyClosure(
+    Store & srcStore, Store & dstStore,
+    const StorePathSet & paths,
+    RepairFlag repair = NoRepair,
+    CheckSigsFlag checkSigs = CheckSigs,
+    SubstituteFlag substitute = NoSubstitute);
+
 /* Remove the temporary roots file for this process.  Any temporary
    root becomes garbage after this point unless it has been registered
    as a (permanent) root. */