This change flips the polarity on parallelism: rather than having a
--serialize flag, we will have a --parallel=P flag, and by default
we will shut off parallelism. We aren't benefiting from it at the
moment (until we implement pulumi/pulumi-fabric#106), and there are
more hidden dependencies in places like AWS Lambdas and Permissions
than I had realized. We may revisit the default, but this allows
us to bite off the messiness of dependsOn only when we benefit from
it. And in any case, the --parallel=P capability will be useful.
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