reflect/protoreflect: rename methods with Type suffix

The protobuf type system uses the word "descriptor" instead of "type".
We should avoid the "type" verbage when we aren't talking about Go types.
The old names are temporarily kept around for compatibility reasons.

Change-Id: Icc99c913528ead011f7a74aa8399d9c5ec6dc56e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/172238
Reviewed-by: Herbie Ong <herbie@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/fileinit/desc.go b/internal/fileinit/desc.go
index 120b7b9..1d1e10a 100644
--- a/internal/fileinit/desc.go
+++ b/internal/fileinit/desc.go
@@ -439,13 +439,19 @@
 func (fd *fieldDesc) HasDefault() bool                           { return fd.defVal.has }
 func (fd *fieldDesc) Default() pref.Value                        { return fd.defVal.get() }
 func (fd *fieldDesc) DefaultEnumValue() pref.EnumValueDescriptor { return fd.defVal.enum }
-func (fd *fieldDesc) OneofType() pref.OneofDescriptor            { return fd.oneofType }
-func (fd *fieldDesc) ExtendedType() pref.MessageDescriptor       { return nil }
-func (fd *fieldDesc) EnumType() pref.EnumDescriptor              { return fd.enumType }
-func (fd *fieldDesc) MessageType() pref.MessageDescriptor        { return fd.messageType }
+func (fd *fieldDesc) Oneof() pref.OneofDescriptor                { return fd.oneofType }
+func (fd *fieldDesc) Extendee() pref.MessageDescriptor           { return nil }
+func (fd *fieldDesc) Enum() pref.EnumDescriptor                  { return fd.enumType }
+func (fd *fieldDesc) Message() pref.MessageDescriptor            { return fd.messageType }
 func (fd *fieldDesc) Format(s fmt.State, r rune)                 { pfmt.FormatDesc(s, r, fd) }
 func (fd *fieldDesc) ProtoType(pref.FieldDescriptor)             {}
 
+// TODO: Remove these methods.
+func (fd *fieldDesc) OneofType() pref.OneofDescriptor      { return fd.Oneof() }
+func (fd *fieldDesc) ExtendedType() pref.MessageDescriptor { return fd.Extendee() }
+func (fd *fieldDesc) EnumType() pref.EnumDescriptor        { return fd.Enum() }
+func (fd *fieldDesc) MessageType() pref.MessageDescriptor  { return fd.Message() }
+
 func (od *oneofDesc) Options() pref.ProtoMessage {
 	return unmarshalOptions(descopts.Oneof, od.options)
 }
@@ -506,10 +512,10 @@
 func (xd *extensionDesc) HasDefault() bool                           { return xd.lazyInit().defVal.has }
 func (xd *extensionDesc) Default() pref.Value                        { return xd.lazyInit().defVal.get() }
 func (xd *extensionDesc) DefaultEnumValue() pref.EnumValueDescriptor { return xd.lazyInit().defVal.enum }
-func (xd *extensionDesc) OneofType() pref.OneofDescriptor            { return nil }
-func (xd *extensionDesc) ExtendedType() pref.MessageDescriptor       { return xd.extendedType }
-func (xd *extensionDesc) EnumType() pref.EnumDescriptor              { return xd.lazyInit().enumType }
-func (xd *extensionDesc) MessageType() pref.MessageDescriptor        { return xd.lazyInit().messageType }
+func (xd *extensionDesc) Oneof() pref.OneofDescriptor                { return nil }
+func (xd *extensionDesc) Extendee() pref.MessageDescriptor           { return xd.extendedType }
+func (xd *extensionDesc) Enum() pref.EnumDescriptor                  { return xd.lazyInit().enumType }
+func (xd *extensionDesc) Message() pref.MessageDescriptor            { return xd.lazyInit().messageType }
 func (xd *extensionDesc) Format(s fmt.State, r rune)                 { pfmt.FormatDesc(s, r, xd) }
 func (xd *extensionDesc) ProtoType(pref.FieldDescriptor)             {}
 func (xd *extensionDesc) ProtoInternal(pragma.DoNotImplement)        {}
@@ -527,6 +533,12 @@
 	return xd.legacyDesc
 }
 
+// TODO: Remove these methods.
+func (xd *extensionDesc) OneofType() pref.OneofDescriptor      { return xd.Oneof() }
+func (xd *extensionDesc) ExtendedType() pref.MessageDescriptor { return xd.Extendee() }
+func (xd *extensionDesc) EnumType() pref.EnumDescriptor        { return xd.Enum() }
+func (xd *extensionDesc) MessageType() pref.MessageDescriptor  { return xd.Message() }
+
 type (
 	serviceDesc struct {
 		baseDesc
@@ -563,14 +575,18 @@
 func (md *methodDesc) Options() pref.ProtoMessage {
 	return unmarshalOptions(descopts.Method, md.options)
 }
-func (md *methodDesc) InputType() pref.MessageDescriptor   { return md.inputType }
-func (md *methodDesc) OutputType() pref.MessageDescriptor  { return md.outputType }
+func (md *methodDesc) Input() pref.MessageDescriptor       { return md.inputType }
+func (md *methodDesc) Output() pref.MessageDescriptor      { return md.outputType }
 func (md *methodDesc) IsStreamingClient() bool             { return md.isStreamingClient }
 func (md *methodDesc) IsStreamingServer() bool             { return md.isStreamingServer }
 func (md *methodDesc) Format(s fmt.State, r rune)          { pfmt.FormatDesc(s, r, md) }
 func (md *methodDesc) ProtoType(pref.MethodDescriptor)     {}
 func (md *methodDesc) ProtoInternal(pragma.DoNotImplement) {}
 
+// TODO: Remove these methods.
+func (md *methodDesc) InputType() pref.MessageDescriptor  { return md.Input() }
+func (md *methodDesc) OutputType() pref.MessageDescriptor { return md.Output() }
+
 type baseDesc struct {
 	parentFile *fileDesc
 	parent     pref.Descriptor