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/github.com/moby/buildkit/sourcepolicy/pb/generate.go

@@ -1,3 +1,3 @@
 package moby_buildkit_v1_sourcepolicy //nolint:revive
 
-//go:generate protoc -I=. --gogofaster_out=plugins=grpc:. policy.proto
+//go:generate protoc -I=. --go_out=paths=source_relative:. --go-grpc_out=paths=source_relative,require_unimplemented_servers=false:. policy.proto

vendor/github.com/moby/buildkit/sourcepolicy/pb/json.go

@@ -1,7 +1,7 @@
 package moby_buildkit_v1_sourcepolicy //nolint:revive
 
 import (
-	"github.com/gogo/protobuf/proto"
+	"github.com/moby/buildkit/util/gogo/proto"
 	"github.com/pkg/errors"
 )
 

vendor/github.com/moby/buildkit/sourcepolicy/pb/policy.proto

@@ -2,6 +2,8 @@ syntax = "proto3";
 
 package moby.buildkit.v1.sourcepolicy;
 
+option go_package = "github.com/moby/buildkit/sourcepolicy/pb;moby_buildkit_v1_sourcepolicy";
+
 // Rule defines the action(s) to take when a source is matched
 message Rule {
 	PolicyAction action = 1; 
@@ -61,4 +63,4 @@ enum MatchType {
 	// REGEX treats the source identifier as a regular expression
 	// With regex matching you can also use match groups to replace values in the destination identifier
 	REGEX = 2;
-}
+}