Partial parsing: duplicate resource error (redux)

[drewbanin]

Describe the bug

Similar to #2055, the --partial-parse flag causes dbt to raise a compilation error for duplicate resources when no such duplicates exist with the following dbt_project.yml config:

...
source-paths: ["models", "tests"]
test-paths: ["tests"]

....

I'm a little bit stumped as to why this is happening, but I can replicate it pretty readily.

Steps To Reproduce

1. Update dbt_project.yml to look include:

source-paths: ["models", "tests"]
test-paths: ["tests"]

2. Add a sql file (select 1 as id) in the tests/ directory

3. run the following commands:

rm -rf target
dbt --partial-parse compile
dbt --partial-parse compile

The second invocation should result in:

Running with dbt=0.15.2-a1
Encountered an error:
Compilation Error
  dbt found two resources with the name "my_test". Since these resources have the same name,
  dbt will be unable to find the correct resource when ref("my_test") is used. To fix this,
  change the name of one of these resources:
  - model.my_project.my_test (tests/my_test.sql)
  - model.my_project.my_test (tests/my_test.sql)

The output of dbt --version:
Using 0.15.1-a2 with this PR included: #2056