[CT-2472] Allow duplicate model names across packages

[MichelleArk]

Current State
During manifest parsing, dbt raises a compilation error if dbt detects two (unversioned) models with the same name:

Compilation Error
  dbt found two models with the name "model_a".
  
  Since these resources have the same name, dbt will be unable to find the correct resource
  when looking for ref("model_a").
  
  To fix this, change the name of one of these resources:
  - model.project_b.model_a (models/model_a.sql)
  - model.project_a.model_a (models/model_a.sql)

This is so that refs originating from any project (project_a, project_b, root_project) don’t have an ambiguous resolution for ref('model_a')

Proposal
To support installing multiple packages as projects, we should relax this uniqueness constraint and allow duplicate model (or other ref-able resource) names across packages. Model name uniqueness should still be enforced within a single project.

ref resolution
Assuming a model named model_a is defined in project_a, and project_b, both of which are installed in a root project:

1. model in project_a has a ref('model_a') → ref resolves to model.project_a.model_a
2. model in project_b has a ref('model_a') → ref resolves to model.project_b.model_a
3. model in root (running) project has a ref('model_a')
   • if it defines its own model_a node → model.root_project.model_a
   • otherwise: raise an error indicating an ambiguous ref

The first two scenarios already work! But the third is broken because dbt resolves “model_a” to an arbitrary project.

** Notes **
Bypassing the initial error can lead to a totally legitimate error aiming to prevent clobbering in the database:

Compilation Error
  dbt found two resources with the database representation ""dbt"."test_arky"."model_a"".
  dbt cannot create two resources with identical database representations. To fix this,
  change the configuration of one of these resources:
  - model.project_b.model_a (models/model_a.sql)
  - model.project_a.model_a (models/model_a.sql)

In this case, the root project should define configurations in its dbt_project.yml to ensure no alias clobbering occurs. For example:

models:
  project_a:
    +schema: project_a
  project_b:
    +schema: project_b