[CT-2424] [Regression] Duplicate Seed or Snapshot across packages fails with: AttributeError: object has no attribute 'version'

[MichelleArk]

Creating a duplicate seed or snapshot across multiple packages currently crashes with:

22:06:04  Traceback (most recent call last):
  File "/Users/michelleark/src/dbt-core/core/dbt/cli/requires.py", line 86, in wrapper
    result, success = func(*args, **kwargs)
  File "/Users/michelleark/src/dbt-core/core/dbt/cli/requires.py", line 71, in wrapper
    return func(*args, **kwargs)
  File "/Users/michelleark/src/dbt-core/core/dbt/cli/requires.py", line 142, in wrapper
    return func(*args, **kwargs)
  File "/Users/michelleark/src/dbt-core/core/dbt/cli/requires.py", line 168, in wrapper
    return func(*args, **kwargs)
  File "/Users/michelleark/src/dbt-core/core/dbt/cli/requires.py", line 215, in wrapper
    return func(*args, **kwargs)
  File "/Users/michelleark/src/dbt-core/core/dbt/cli/requires.py", line 242, in wrapper
    manifest = ManifestLoader.get_full_manifest(
  File "/Users/michelleark/src/dbt-core/core/dbt/parser/manifest.py", line 229, in get_full_manifest
    _check_manifest(manifest, config)
  File "/Users/michelleark/src/dbt-core/core/dbt/parser/manifest.py", line 1131, in _check_manifest
    _check_resource_uniqueness(manifest, config)
  File "/Users/michelleark/src/dbt-core/core/dbt/parser/manifest.py", line 1109, in _check_resource_uniqueness
    if existing_node is not None and existing_node.version is None:
AttributeError: 'SeedNode' object has no attribute 'version'

The expected behaviour should instead be:

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

This is because an unsafe access to the version attribute is attempted in _check_resource_uniqueness: https://github.com/dbt-labs/dbt-core/blob/main/core/dbt/parser/manifest.py#L1109