Recalculate metric depends_on in partial parsing

[jtcohen6]

In partial parsing, when a metric's model is updated, the depends_on for metrics is not updated, even though refs is updated.

reproduction steps

Create any models model_a + model_b, then create a metric like:

# models/whatever.yml
version: 2
metrics:
  - model: ref('model_a')
    name: my_metric
    label: "My Metric"
    type: count
    sql: "*"
    timestamp: my_date
    time_grains: [day]

Good on the first go:

$ dbt --partial-parse ls --resource-type metric --output json --output-keys name,refs,depends_on
...
{"name": "my_metric", "depends_on": {"macros": [], "nodes": ["model.testy.model_a"]}, "refs": [["model_a"]]}

Then, edit the model property to be ref('model_b') instead.

$ dbt --partial-parse ls --resource-type metric --output json --output-keys name,refs,depends_on
...
{"name": "my_metric", "depends_on": {"macros": [], "nodes": []}, "refs": [["model_b"]]}

$ dbt --no-partial-parse ls --resource-type metric --output json --output-keys name,refs,depends_on
{"name": "my_metric", "depends_on": {"macros": [], "nodes": ["model.testy.model_b"]}, "refs": [["model_b"]]}