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/libstore/remote-store.hh

@@ -150,6 +150,9 @@ public:
 
     std::string getUri() override;
 
+    static std::vector<std::string> uriPrefixes()
+    { return {"unix"}; }
+
     bool sameMachine() override
     { return true; }