Ensure all store types support "real" URIs

In particular `local://<path>` and `unix://` (without any path) now
work, and mean the same things as `local` and `daemon`, respectively. We
thus now have the opportunity to desguar `local` and `daemon` early.

This will allow me to make a change to
https://github.com/NixOS/nix/pull/9839 requested during review to
desugar those earlier.

Co-authored-by: Théophane Hufschmitt <7226587+thufschmitt@users.noreply.github.com>

src/libstore/http-binary-cache-store.cc

@@ -39,15 +39,20 @@ private:
 public:
 
     HttpBinaryCacheStore(
-        const std::string & scheme,
-        const Path & _cacheUri,
+        std::string_view scheme,
+        PathView _cacheUri,
         const Params & params)
         : StoreConfig(params)
         , BinaryCacheStoreConfig(params)
         , HttpBinaryCacheStoreConfig(params)
         , Store(params)
         , BinaryCacheStore(params)
-        , cacheUri(scheme + "://" + _cacheUri)
+        , cacheUri(
+            std::string { scheme }
+            + "://"
+            + (!_cacheUri.empty()
+                ? _cacheUri
+                : throw UsageError("`%s` Store requires a non-empty authority in Store URL", scheme)))
     {
         while (!cacheUri.empty() && cacheUri.back() == '/')
             cacheUri.pop_back();