Make the store plugins more introspectable

Directly register the store classes rather than a function to build an
instance of them.
This gives the possibility to introspect static members of the class or
choose different ways of instantiating them.

src/libexpr/flake/lockfile.hh

@@ -6,7 +6,7 @@
 
 namespace nix {
 class Store;
-struct StorePath;
+class StorePath;
 }
 
 namespace nix::flake {