Looper resolves pipeline interface paths differently if multiple interfaces are supplied

[donaldcampbelljr]

Noticing this as I refactor Looper tests.

pep_config: project/project_config.yaml
output_dir: "results"
pipeline_interfaces:
  sample:
    - pipeline/pipeline_interface1_sample.yaml
    - pipeline/pipeline_interface2_sample.yaml

pep_config: project/project_config.yaml
output_dir: "results"
pipeline_interfaces:
  sample: pipeline/pipeline_interface1_sample.yaml

The piface path is resolved differently if two interfaces are given under sample or project.