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/store-api.cc

@@ -300,9 +300,9 @@ Store::Store(const Store::Config & config)
     assertLibStoreInitialized();
 }
 
-std::string StoreConfig::getUri() const
+StoreReference StoreConfig::getReference() const
 {
-    return "";
+    return {.variant = StoreReference::Auto{}};
 }
 
 bool Store::PathInfoCacheValue::isKnownNow()