all: move v1 types over to the v2 repository

As a goal, v2 should not depend on v1. As another step towards that end,
we move all the types that used to be in the v1 protoapi package over to v2.

For now, we place MessageV1, ExtensionRangeV1, and ExtensionDescV1
in runtime/protoiface since these are types that generated messages will
probably have to reference forever. An alternative location could be
reflect/protoreflect, but it seems unfortunate to have to dirty the
namespace of that package with these types.

We move ExtensionFieldV1, ExtensionFieldsV1, and ExtensionFieldsOf
to internal/impl, since these are related to the implementation of a
generated message.

Since moving these types from v1 to v2 implies that the v1 protoapi
package is useless, we update all usages of v1 protoapi in the v2
repository to point to the relevant v2 type or functionality.

CL/168538 is the corresponding change to alter v1.
There will be a temporary build failure as it is not possible
to submit CL/168519 and CL/168538 atomically.

Change-Id: Ide4025c1b6af5b7f0696f4b65b988b4d10a50f0b
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/168519
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/internal/testprotos/test/test.pb.go b/internal/testprotos/test/test.pb.go
index fe48501..24ef243 100644
--- a/internal/testprotos/test/test.pb.go
+++ b/internal/testprotos/test/test.pb.go
@@ -6,6 +6,7 @@
 import (
 	proto "github.com/golang/protobuf/proto"
 	protoreflect "github.com/golang/protobuf/v2/reflect/protoreflect"
+	protoiface "github.com/golang/protobuf/v2/runtime/protoiface"
 	protoimpl "github.com/golang/protobuf/v2/runtime/protoimpl"
 	reflect "reflect"
 )
@@ -1213,10 +1214,10 @@
 var xxx_messageInfo_TestReservedFields proto.InternalMessageInfo
 
 type TestAllExtensions struct {
-	XXX_NoUnkeyedLiteral         struct{} `json:"-"`
-	proto.XXX_InternalExtensions `json:"-"`
-	XXX_unrecognized             []byte `json:"-"`
-	XXX_sizecache                int32  `json:"-"`
+	XXX_NoUnkeyedLiteral   struct{}                    `json:"-"`
+	XXX_InternalExtensions protoimpl.ExtensionFieldsV1 `json:"-"`
+	XXX_unrecognized       []byte                      `json:"-"`
+	XXX_sizecache          int32                       `json:"-"`
 }
 
 func (m *TestAllExtensions) ProtoReflect() protoreflect.Message {
@@ -1231,12 +1232,12 @@
 	return xxx_File_test_test_proto_rawdesc_gzipped, []int{4}
 }
 
-var extRange_TestAllExtensions = []proto.ExtensionRange{
+var extRange_TestAllExtensions = []protoiface.ExtensionRangeV1{
 	{Start: 1, End: 536870911},
 }
 
 // Deprecated: Use TestAllExtensions.ProtoReflect.Type.ExtensionRanges instead.
-func (*TestAllExtensions) ExtensionRangeArray() []proto.ExtensionRange {
+func (*TestAllExtensions) ExtensionRangeArray() []protoiface.ExtensionRangeV1 {
 	return extRange_TestAllExtensions
 }
 
@@ -1595,7 +1596,7 @@
 	return 0
 }
 
-var xxx_File_test_test_proto_extDescs = []proto.ExtensionDesc{
+var xxx_File_test_test_proto_extDescs = []protoiface.ExtensionDescV1{
 	{
 		ExtendedType:  (*TestAllExtensions)(nil),
 		ExtensionType: (*int32)(nil),