Use built_value for serialization.

[davidmorgan]

For #3811.

Remove hand-coded serialization of AssetNode, use built_value's generated code.

Store globs as their String pattern instead of as a Glob to save having to serialize the Glob type.

Before this PR the serialization was dropping half of the graph data on serialization and reconstructing it on deserialization, adding back inputs by reversing the outputs links on load. Stop doing that and just serialize+deserialize unchanged. This turns out to need one fix for correctness: track one input in build_impl.dart that was being restored on load.

The output is quite a lot larger than before, for the 1000 node benchmark the serialized graph is now 228Mb(!) whereas previously it was 20Mb(!). This is because the previous serialization code optimizes by replacing AssetIds with one int each used to refer to that ID throughout the JSON. The 10x saving is nice, but we need to "dedupe" whole subgraphs, not just individual IDs, in the end the graph for that benchmark should be well under 1Mb.

This restores a good chunk of the performance lost in the previous PR, the details are not important as we are obviously some way from the desired final state, but here are the numbers in case anyone wants them

json_serializable
shape,libraries,clean/ms,no changes/ms,incremental/ms
loop,1,22922,4294,5286
loop,100,24904,4455,8344
loop,250,30565,5193,13646
loop,500,53779,7798,25373
loop,750,105386,12212,45280
loop,1000,187782,20752,74371

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