Move dep override file

Move the file into a package that no one imports. That way consumers of
dep aren't forced to pick up dependencies listed in that file.

pkg/dep.go → pkg/dep/dep.go

@@ -1,4 +1,4 @@
-package pkg
+package dep
 
 // These imports turn transitive dependencies into direct dependencies
 // so that we can control then using "constraint" in our dep manifest.