all: rename ExtensionType Descriptor method to TypeDescriptor (1/2)

Descriptor methods generally return a Descriptor with no Go type
information. ExtensionType's Descriptor is an exception, returning an
ExtensionTypeDescriptor containing both the proto descriptor and a
reference back to the ExtensionType. The pure descriptor is accessed
by xt.Descriptor().Descriptor().

Rename ExtensionType's Descriptor method to TypeDescriptor to make it
clear that it behaves a bit differently.

Change 1/2: Add the TypeDescriptor method and deprecate Descriptor.

Change-Id: I1806095044d35a474d60f94d2a28bdf528f12238
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/192139
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/impl/extension.go b/internal/impl/extension.go
index 1ba473d..2ac8b7b 100644
--- a/internal/impl/extension.go
+++ b/internal/impl/extension.go
@@ -119,6 +119,10 @@
 	return xi.goType
 }
 func (xi *ExtensionInfo) Descriptor() pref.ExtensionTypeDescriptor {
+	// TODO: Remove.
+	return xi.TypeDescriptor()
+}
+func (xi *ExtensionInfo) TypeDescriptor() pref.ExtensionTypeDescriptor {
 	if atomic.LoadUint32(&xi.init) == extensionInfoUninitialized {
 		xi.lazyInitSlow()
 	}