Gets the packets in order: one unnamed packet when the entry is a stream, else one per name and stream pair.
public IReadOnlyList<PdfXfaPacket> Packets { get; }
ISO 32000-2 Annex K: the array form is [name1 stream1 name2 stream2 ...], names being text strings such as
template and datasets. A pair whose name is not a string or whose value is not a stream, and an odd element
left over, are skipped with an XfaPacketsInvalid diagnostic.