Rewrite StoreConfig::getUri in terms of new StoreConfig::getReference

Rather than having store implementations return a free-form URI string,
have them return a `StoreReference`. This reflects that fact that this
method is supposed to invert `resolveStoreConfig`, which goes from a
`StoreReference` to some `StoreConfig` concrete derived class (based on
the registry).

`StoreConfig::getUri` is kept only as a convenience for the common case
that we want to immediately render the `StoreReference`.

A few tests were changed to use `local://` not `local`, since
`StoreReference` does not encode the `local` and `daemon` shorthands
(and instead desugars them to `local://` and `unix://` right away). I
think that is fine. `local` and `daemon` still work as input.

src/libstore/dummy-store.cc

@@ -33,9 +33,14 @@ struct DummyStoreConfig : public std::enable_shared_from_this<DummyStoreConfig>,
 
     ref<Store> openStore() const override;
 
-    std::string getUri() const override
+    StoreReference getReference() const override
     {
-        return *uriSchemes().begin();
+        return {
+            .variant =
+                StoreReference::Specified{
+                    .scheme = *uriSchemes().begin(),
+                },
+        };
     }
 };