all: bump go.mod version and drop compatibility shims

Also, remove the legacy import annotations.

Fixes golang/go#68147

Change-Id: Ibfcc9322f27224c0ba92ea42cd56912a7d8783fd
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/594256
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

nacl/box/box.go

@@ -35,7 +35,7 @@ Anonymous sealing/opening is an extension of NaCl defined by and interoperable
 with libsodium:
 https://libsodium.gitbook.io/doc/public-key_cryptography/sealed_boxes.
 */
-package box // import "golang.org/x/crypto/nacl/box"
+package box
 
 import (
 	cryptorand "crypto/rand"

nacl/secretbox/secretbox.go

@@ -32,7 +32,7 @@ chunk size.
 
 This package is interoperable with NaCl: https://nacl.cr.yp.to/secretbox.html.
 */
-package secretbox // import "golang.org/x/crypto/nacl/secretbox"
+package secretbox
 
 import (
 	"golang.org/x/crypto/internal/alias"