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-store.cc

@@ -439,15 +439,15 @@ LocalStore::~LocalStore()
     }
 }
 
-std::string LocalStoreConfig::getUri() const
+StoreReference LocalStoreConfig::getReference() const
 {
-    std::ostringstream oss;
-    oss << *uriSchemes().begin();
-    auto queryParams = getQueryParams();
-    if (!queryParams.empty())
-        oss << "?";
-    oss << encodeQuery(queryParams);
-    return std::move(oss).str();
+    return {
+        .variant =
+            StoreReference::Specified{
+                .scheme = *uriSchemes().begin(),
+            },
+        .params = getQueryParams(),
+    };
 }
 
 int LocalStore::getSchema()