Feature: Add excluded_packages configuration to prevent specific nested packages from being injected

[nicholasyager]

Description and Motivation

In some circumstances, a user may want to prevent a package imported into an upstream project from being injected into a downstream project. One example of this is dbt_project_evaluator, which provides valuable test results about project health and architectural soundness. It's not uncommon for teams to import the dbt_project_evaluator package in all of their projects.

flowchart TB
    project_a --> project_b
    dbt_project_evaluator --> project_a
    dbt_project_evaluator --> project_b

Loading

Unfortunately, dbt-core's access control model is namespaced on project name and not lineage, so project_b will incidentally inject dbt_project_evaluator models from project_a despite the models being marked as protected.

To work around this issue, this PR adds an excluded_packages configuration in the ManifestReference to inform dbt-loom to exclude all resources from a specific upstream project that come from a given package.

For example, the follow configuration will exclude all dbt_project_evaluator resources from the revenue project, clearing the way for this downstream project's version of the package.

manifests:
  - name: revenue
    type: file
    config:
      path: ../revenue/target/manifest.json
    excluded_packages:
      - dbt_project_evaluator

Resolves: #93