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/ssh-store-config.hh

@@ -24,6 +24,29 @@ struct CommonSSHStoreConfig : virtual StoreConfig
           to be used on the remote machine. The default is `auto`
           (i.e. use the Nix daemon or `/nix/store` directly).
         )"};
+
+    /**
+     * The `parseURL` function supports both IPv6 URIs as defined in
+     * RFC2732, but also pure addresses. The latter one is needed here to
+     * connect to a remote store via SSH (it's possible to do e.g. `ssh root@::1`).
+     *
+     * This function now ensures that a usable connection string is available:
+     *
+     * - If the store to be opened is not an SSH store, nothing will be done.
+     *
+     * - If the URL looks like `root@[::1]` (which is allowed by the URL parser and probably
+     *   needed to pass further flags), it
+     *   will be transformed into `root@::1` for SSH (same for `[::1]` -> `::1`).
+     *
+     * - If the URL looks like `root@::1` it will be left as-is.
+     *
+     * - In any other case, the string will be left as-is.
+     *
+     * Will throw an error if `connStr` is empty too.
+     */
+    static std::string extractConnStr(
+        std::string_view scheme,
+        std::string_view connStr);
 };
 
 }