Remove the 'traverse_serial' fields which were added in 2.1.1.
Use the 'serial' field to track the current traversal.
Stop using 'identifier' to sort packages in the flattened solution.
Directly construct the flattened solution by a specific walk which
also preserves the relative order in Requires and Requires.private.
The topological sort is a single list, so don't fill requires_private.
Purely private dependencies are marked in dependency flags.
The ancestor flag is a pkg property, not a client property.
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