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>

acme/http.go

@@ -15,6 +15,7 @@ import (
 	"io"
 	"math/big"
 	"net/http"
+	"runtime/debug"
 	"strconv"
 	"strings"
 	"time"
@@ -271,9 +272,27 @@ func (c *Client) httpClient() *http.Client {
 }
 
 // packageVersion is the version of the module that contains this package, for
-// sending as part of the User-Agent header. It's set in version_go112.go.
+// sending as part of the User-Agent header.
 var packageVersion string
 
+func init() {
+	// Set packageVersion if the binary was built in modules mode and x/crypto
+	// was not replaced with a different module.
+	info, ok := debug.ReadBuildInfo()
+	if !ok {
+		return
+	}
+	for _, m := range info.Deps {
+		if m.Path != "golang.org/x/crypto" {
+			continue
+		}
+		if m.Replace == nil {
+			packageVersion = m.Version
+		}
+		break
+	}
+}
+
 // userAgent returns the User-Agent header value. It includes the package name,
 // the module version (if available), and the c.UserAgent value (if set).
 func (c *Client) userAgent() string {

acme/version_go112.go

@@ -1,27 +0,0 @@
-// 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 go1.12
-
-package acme
-
-import "runtime/debug"
-
-func init() {
-	// Set packageVersion if the binary was built in modules mode and x/crypto
-	// was not replaced with a different module.
-	info, ok := debug.ReadBuildInfo()
-	if !ok {
-		return
-	}
-	for _, m := range info.Deps {
-		if m.Path != "golang.org/x/crypto" {
-			continue
-		}
-		if m.Replace == nil {
-			packageVersion = m.Version
-		}
-		break
-	}
-}