Multi-project collaboration - Milestone 2

[jtcohen6]

Namespacing & XP ref

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1. Namespacing: models with same name across projects/packages #3571
   content dbt-core v1.6 improvement priority: high +4
   
2. Namespacing: scope & resolution changes for vars & macros #3572
   content dbt-core v1.6 improvement priority: high +4
   
3. packages can be specified in dependencies.yml #3550
   content dbt-core v1.6 improvement priority: high +4
   
4. Model access: enforce_access for packages #3573
   blocked_by_dev content dbt-core v1.6 improvement priority: high +5
   
5. XP ref: explanation; cycle detection; comparison to packages #3574
   content dbt-core v1.6 improvement priority: high +4
   
6. Add restrict-access to dbt_project.yml #3632
   content dbt-core v1.6 improvement +3
   

Model governance: UX improvements

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1. deprecation_date for model versions #3357
   content dbt-core v1.6 +2
   
2. Expanded deprecation_date docs: motivation, examples, tie-in to model governance #3745
   content dbt-core v1.6 improvement +3
   
3. Model contracts: include removal/update of constraints in set of breaking changes #3570
   content dbt-core v1.6 improvement priority: high +4
   
4. Model contracts: matrix showing which constraints are supported/enforced by data warehouse #3282
   content dbt Core dbt-core v1.5 improvement +4
   
5. Add example of colum-level constraint on a nested column in BigQuery #3454
   content dbt Core dbt-core v1.5 improvement +4
   
6. Models with 'ephemeral' materialization cannot have access:public #3475
   content dbt-core v1.6 improvement +3
   
7. Model contracts: users must specify numeric scale on Snowflake #3569
   content dbt-core v1.5 improvement priority: high +4
   
8. Tie in breaking changes to model contracts under governance #3496
   content dbt-core v1.5 improvement +3
9. Model contracts for incremental models: on_schema_change can be 'fail' in addition to 'append_new_columns' #3649
   content dbt-core v1.5 improvement +3