all: handle Go 1.18 code on Go 1.16 AppEngine using new internal/back…

…port/go/*

The immediate need was go/build, because the Go 1.16 go/build rejects
code that has //go:build without // +build, as Go 1.18 code now does.

But displaying code using generics would also have failed, for inability
to parse that code. Add the full go/ast, go/doc, go/format, go/parser,
go/printer, go/scanner, and go/token to fix those failures, which
would become more frequent as generics are used more inside the
standard library.

The code is all from the go1.18 tag of the Go repo, post-processed to
rewrite imports and apply gofmt -w -r 'any -> interface{}' to keep it
building with the other backports and with Go 1.16 syntax.

For golang/go#51686.

Change-Id: I1e14f4634d8bc09bdaa04c014eadb1be97ea5047
Reviewed-on: https://go-review.googlesource.com/c/website/+/393194
Trust: Russ Cox <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

cmd/golangorg/server.go

@@ -14,7 +14,6 @@ import (
 	"errors"
 	"flag"
 	"fmt"
-	"go/format"
 	"io/fs"
 	"io/ioutil"
 	"log"
@@ -32,6 +31,7 @@ import (
 	"cloud.google.com/go/datastore"
 	"golang.org/x/build/repos"
 	"golang.org/x/website"
+	"golang.org/x/website/internal/backport/go/format"
 	"golang.org/x/website/internal/backport/html/template"
 	"golang.org/x/website/internal/blog"
 	"golang.org/x/website/internal/codewalk"

internal/api/api_test.go

@@ -5,10 +5,11 @@
 package api
 
 import (
-	"go/build"
 	"os"
 	"runtime"
 	"testing"
+
+	"golang.org/x/website/internal/backport/go/build"
 )
 
 func TestParseVersionRow(t *testing.T) {