zip: Add fs.FS support for Go 1.16+ (#432)

* zip: Add fs.FS support for Go 1.16+

Change writer api to match stdlib as well.

`func (w *Writer) Copy(name string, src *File) error` is changed to `func (w *Writer) Copy(src *File) error` to match the stdlib.

zip/internal/obscuretestdata/obscuretestdata.go

@@ -0,0 +1,53 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.16
+// +build go1.16
+
+// Package obscuretestdata contains functionality used by tests to more easily
+// work with testdata that must be obscured primarily due to
+// golang.org/issue/34986.
+package obscuretestdata
+
+import (
+	"encoding/base64"
+	"io"
+	"os"
+)
+
+// DecodeToTempFile decodes the named file to a temporary location.
+// If successful, it returns the path of the decoded file.
+// The caller is responsible for ensuring that the temporary file is removed.
+func DecodeToTempFile(name string) (path string, err error) {
+	f, err := os.Open(name)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+
+	tmp, err := os.CreateTemp("", "obscuretestdata-decoded-")
+	if err != nil {
+		return "", err
+	}
+	if _, err := io.Copy(tmp, base64.NewDecoder(base64.StdEncoding, f)); err != nil {
+		tmp.Close()
+		os.Remove(tmp.Name())
+		return "", err
+	}
+	if err := tmp.Close(); err != nil {
+		os.Remove(tmp.Name())
+		return "", err
+	}
+	return tmp.Name(), nil
+}
+
+// ReadFile reads the named file and returns its decoded contents.
+func ReadFile(name string) ([]byte, error) {
+	f, err := os.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return io.ReadAll(base64.NewDecoder(base64.StdEncoding, f))
+}