Enforce a single Noir contract per package

[TomAFrench]

Problem

We currently support an unlimited number of contracts within a single Noir package. We've decided that we'd like there to be a single contract per package instead.

Happy Case

Multiple contracts should result in a parsing error. We can then replace the functions to pull contracts from the defmap to instead return a single contract.

As there's only a single contract we can also remove the name field from contract <name> {} however we could maintain this and simply ignore it.

Alternatives Considered

No response

Additional Context

No response

Would you like to submit a PR for this Issue?

No

Support Needs

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