export dbt-staging-sql error on multi models contracts

[cedric-orange]

When there is more than one model in contract, datacontract export --format dbt-staging-sql --model name is not usable.

With correct model name:

$ datacontract export ./datacontract_50D_50D.yml --format dbt-staging-sql --model PNA
Export to dbt-staging-sql currently only works with exactly one model in the data contract.Please specify the model name.

With an incorrect model name:

$ datacontract export ./datacontract_50D_50D.yml --format dbt-staging-sql --model models.PNA
RuntimeError: Model model.PNA not found in the data contract. Available models: ['FAC', 'PNA']