This change adds support for serializing snapshots in MuGL, per the
design document in docs/design/mugl.md. At the moment, it is only
exposed from the `mu plan` command, which allows you to specify an
output location using `mu plan --output=file.json` (or `-o=file.json`
for short). This serializes the snapshot with monikers, resources,
and so on. Deserialization is not yet supported; that comes next.
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