dbt importer leaves contract id as placeholder `my-data-contract`

[jschoedl]

datacontract import --format dbt --source manifest.json

Actual: imported contract has id: my-data-contract regardless of the dbt project.

Expected: id derived from the dbt manifest's project_name (matching what's already done for name).

• Relevant file: datacontract/imports/dbt_importer.py (import_dbt_manifest)