go/internal/xurl: New package (draft)

With xurl.ParseQuery being simpler analog of url.ParseQuery.
Simpler: It returns regular map instead of url.Values by not allowing duplicates.

go/internal/xurl/xurl.go

+// Copyright (C) 2020-2021  Nexedi SA and Contributors.
+//                          Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+// Package xurl complements standard package net/url.
+package xurl
+
+import (
+	"fmt"
+	"net/url"
+)
+
+// ParseQuery parses query string into regular map.
+// see also url.ParseQuery
+func ParseQuery(query string) (map[string]string, error) {
+	qv, err := url.ParseQuery(query)
+	if err != nil {
+		return nil, err
+	}
+	q := map[string]string{}
+	for k, vv := range qv {
+		if len(vv) == 0 {
+			return nil, fmt.Errorf("parameter %q without value", k)
+		}
+		if len(vv) != 1 {
+			return nil, fmt.Errorf("duplicate parameter %q ", k)
+		}
+		q[k] = vv[0]
+	}
+	return q, nil
+}