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/internal/impl/legacy_file.go b/internal/impl/legacy_file.go
index b1293c6..bccaede 100644
--- a/internal/impl/legacy_file.go
+++ b/internal/impl/legacy_file.go
@@ -56,7 +56,7 @@
 		panic(err)
 	}
 
-	fd := filedesc.DescBuilder{
+	fd := filedesc.Builder{
 		RawDescriptor: b2,
 		FileRegistry:  resolverOnly{protoregistry.GlobalFiles}, // do not register back to global registry
 	}.Build().File