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/filedesc/desc_test.go b/internal/filedesc/desc_test.go
index ab349c7..08930a2 100644
--- a/internal/filedesc/desc_test.go
+++ b/internal/filedesc/desc_test.go
@@ -198,14 +198,14 @@
 	if err != nil {
 		t.Fatalf("proto.Marshal() error: %v", err)
 	}
-	fd2 := filedesc.DescBuilder{RawDescriptor: b}.Build().File
+	fd2 := filedesc.Builder{RawDescriptor: b}.Build().File
 
 	tests := []struct {
 		name string
 		desc pref.FileDescriptor
 	}{
 		{"protodesc.NewFile", fd1},
-		{"filedesc.DescBuilder.Build", fd2},
+		{"filedesc.Builder.Build", fd2},
 	}
 	for _, tt := range tests {
 		tt := tt