os: Drop support for go 1.15

1.15 specific files deleted.
1.16 specific files folded carefully into generic files, with goal of reducing diff with upstream.
Follows upstream 1.16 in making PathError etc. be aliases for the same errors in io/fs.

This fixes #2817 and lets us add io/ioutil to "make test-tinygo" on linux and mac.

src/os/dir.go

@@ -1,6 +1,3 @@
-//go:build go1.16
-// +build go1.16
-
 // Copyright 2016 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.