cmd/protoc-gen-go, internal/filetype: clean up EnumType construction

Drop the dependency from generated files on prototype.Enum: Generated
code should only depend on runtime/proto{iface,impl}.

Drop the Enums, Messages, and Extensions returns form
filetype.Builder.Build. Of these, only Enums was used by generated code.
Change the generated init function to pass the builder a slice of values
to fill in (as is done for messages and extensions).

Remove the filetype dependency on prototype in preparation for
eventually dropping the prototype package entirely.

Change-Id: I28a3420f5dfcc13fed531a64ef07b9afddfd9d55
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189200
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/runtime/protoimpl/impl.go b/runtime/protoimpl/impl.go
index b419ff6..10ce301 100644
--- a/runtime/protoimpl/impl.go
+++ b/runtime/protoimpl/impl.go
@@ -23,6 +23,7 @@
 type (
 	DescBuilder      = filedesc.Builder
 	TypeBuilder      = filetype.Builder
+	EnumInfo         = filetype.EnumInfo
 	Pointer          = impl.Pointer
 	MessageInfo      = impl.MessageInfo
 	MessageState     = impl.MessageState