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/local-overlay-store.cc

@@ -23,6 +23,16 @@ ref<Store> LocalOverlayStoreConfig::openStore() const
         ref{std::dynamic_pointer_cast<const LocalOverlayStoreConfig>(shared_from_this())});
 }
 
+StoreReference LocalOverlayStoreConfig::getReference() const
+{
+    return {
+        .variant =
+            StoreReference::Specified{
+                .scheme = *uriSchemes().begin(),
+            },
+    };
+}
+
 Path LocalOverlayStoreConfig::toUpperPath(const StorePath & path) const
 {
     return upperLayer + "/" + path.to_string();