protobuf: remove gogoproto

Removes gogo/protobuf from buildx and updates to a version of
moby/buildkit where gogo is removed.

This also changes how the proto files are generated. This is because
newer versions of protobuf are more strict about name conflicts. If two
files have the same name (even if they are relative paths) and are used
in different protoc commands, they'll conflict in the registry.

Since protobuf file generation doesn't work very well with
`paths=source_relative`, this removes the `go:generate` expression and
just relies on the dockerfile to perform the generation.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

vendor/golang.org/x/net/http2/http2.go

@@ -17,6 +17,7 @@ package http2 // import "golang.org/x/net/http2"
 
 import (
 	"bufio"
+	"context"
 	"crypto/tls"
 	"fmt"
 	"io"
@@ -26,6 +27,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"time"
 
 	"golang.org/x/net/http/httpguts"
 )
@@ -210,12 +212,6 @@ type stringWriter interface {
 	WriteString(s string) (n int, err error)
 }
 
-// A gate lets two goroutines coordinate their activities.
-type gate chan struct{}
-
-func (g gate) Done() { g <- struct{}{} }
-func (g gate) Wait() { <-g }
-
 // A closeWaiter is like a sync.WaitGroup but only goes 1 to 0 (open to closed).
 type closeWaiter chan struct{}
 
@@ -383,3 +379,14 @@ func validPseudoPath(v string) bool {
 // makes that struct also non-comparable, and generally doesn't add
 // any size (as long as it's first).
 type incomparable [0]func()
+
+// synctestGroupInterface is the methods of synctestGroup used by Server and Transport.
+// It's defined as an interface here to let us keep synctestGroup entirely test-only
+// and not a part of non-test builds.
+type synctestGroupInterface interface {
+	Join()
+	Now() time.Time
+	NewTimer(d time.Duration) timer
+	AfterFunc(d time.Duration, f func()) timer
+	ContextWithTimeout(ctx context.Context, d time.Duration) (context.Context, context.CancelFunc)
+}