all: drop reflect/prototype package

Remove the remaining uses of the prototype package.

The most significant change is to impl.MessageInfo, which now directly
implements the MessageType interface. This involves two notable changes
to exported fields of MessageInfo:

  - PBType is now Desc.
  - GoType is now GoReflectType. (Name changed to avoid a conflict with
    the GoType method of the MessageType interface.)

Fixes golang/protobuf#911

Change-Id: Ie2aa4766d6887ceaa9cf06b1f109aa6e6e2a208f
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189340
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/impl/legacy_enum.go b/internal/impl/legacy_enum.go
index ffbdbeb..279baa9 100644
--- a/internal/impl/legacy_enum.go
+++ b/internal/impl/legacy_enum.go
@@ -13,7 +13,6 @@
 	"google.golang.org/protobuf/internal/filedesc"
 	"google.golang.org/protobuf/reflect/protoreflect"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
-	"google.golang.org/protobuf/reflect/prototype"
 )
 
 // legacyWrapEnum wraps v as a protoreflect.Enum,
@@ -35,18 +34,10 @@
 
 	// Slow-path: derive enum descriptor and initialize EnumType.
 	var et pref.EnumType
-	var m sync.Map // map[protoreflect.EnumNumber]proto.Enum
 	ed := LegacyLoadEnumDesc(t)
-	et = &prototype.Enum{
-		EnumDescriptor: ed,
-		NewEnum: func(n pref.EnumNumber) pref.Enum {
-			if e, ok := m.Load(n); ok {
-				return e.(pref.Enum)
-			}
-			e := &legacyEnumWrapper{num: n, pbTyp: et, goTyp: t}
-			m.Store(n, e)
-			return e
-		},
+	et = &legacyEnumType{
+		desc:   ed,
+		goType: t,
 	}
 	if et, ok := legacyEnumTypeCache.LoadOrStore(t, et); ok {
 		return et.(pref.EnumType)
@@ -54,6 +45,27 @@
 	return et
 }
 
+type legacyEnumType struct {
+	desc   pref.EnumDescriptor
+	goType reflect.Type
+	m      sync.Map // map[protoreflect.EnumNumber]proto.Enum
+}
+
+func (t *legacyEnumType) New(n pref.EnumNumber) pref.Enum {
+	if e, ok := t.m.Load(n); ok {
+		return e.(pref.Enum)
+	}
+	e := &legacyEnumWrapper{num: n, pbTyp: t, goTyp: t.goType}
+	t.m.Store(n, e)
+	return e
+}
+func (t *legacyEnumType) GoType() reflect.Type {
+	return t.goType
+}
+func (t *legacyEnumType) Descriptor() pref.EnumDescriptor {
+	return t.desc
+}
+
 type legacyEnumWrapper struct {
 	num   pref.EnumNumber
 	pbTyp pref.EnumType