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.cc

@@ -0,0 +1,24 @@
+#include <regex>
+
+#include "ssh-store-config.hh"
+
+namespace nix {
+
+std::string CommonSSHStoreConfig::extractConnStr(std::string_view scheme, std::string_view _connStr)
+{
+    if (_connStr.empty())
+        throw UsageError("`%s` store requires a valid SSH host as the authority part in Store URI", scheme);
+
+    std::string connStr{_connStr};
+
+    std::smatch result;
+    static std::regex v6AddrRegex("^((.*)@)?\\[(.*)\\]$");
+
+    if (std::regex_match(connStr, result, v6AddrRegex)) {
+        connStr = result[1].matched ? result.str(1) + result.str(3) : result.str(3);
+    }
+
+    return connStr;
+}
+
+}