Disallow certain CUE constructs within lineage (schema) declarations

[sdboyer]

There are some logical constructs in CUE that make analysis more complicated, and we'll probably have a much easier time creating the invariants if we just disallow their use within schema declarations. Here's a preliminary list:

• if statements
• comprehensions? def yes if composition logic (Introduce compositional Lineages #8) can be kept entirely outside the schema itself and unified in when called
• aliases? no specific reason to do this apart from it being suggestive that people are being too fancy

Cats don't like going back in bags. Better to err on the side of being restrictive initially, then open up later.

Tasks

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1. disallow if statements in schema definitions #146
   prio/low readiness/prod +2
2. disallow comprehensions in schema definitions #147
   prio/low readiness/prod +2
3. disallow aliases in schema definitions (TBD)

[verdverm]

fyi, Marcel has said that there will probably be a CUE native way to do this in the future. Something to keep an eye on, not sure if there is an issue yet