From the docs for packages.{LoadFiles, LoadSyntax} -
https://godoc.org/golang.org/x/tools/go/packages#pkg-constants
// Deprecated: LoadFiles exists for historical compatibility
// and should not be used. Please directly specify the needed
// fields using the Need values.
// ...
// Deprecated: LoadSyntax exists for historical compatibility
// and should not be used. Please directly specify the needed
// fields using the Need values.