internal/filedesc, internal/filetype: rename {Desc,Type}Builder as Builder

Reduce the stutter in the name since the type of Builder
is obvious from the package it is from.

Change-Id: I0046a5122717536cc6bb5ebdb32b67a1560cfc23
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189020
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/runtime/protoimpl/impl.go b/runtime/protoimpl/impl.go
index 43407b1..775f920 100644
--- a/runtime/protoimpl/impl.go
+++ b/runtime/protoimpl/impl.go
@@ -62,8 +62,8 @@
 	// being a compilation failure (guaranteed by the Go specification).
 	EnforceVersion uint
 
-	DescBuilder      = filedesc.DescBuilder
-	TypeBuilder      = filetype.TypeBuilder
+	DescBuilder      = filedesc.Builder
+	TypeBuilder      = filetype.Builder
 	Pointer          = impl.Pointer
 	MessageInfo      = impl.MessageInfo
 	MessageState     = impl.MessageState