Allow non-root-relative paths for Source Resoruces

[kokorin]

Sources require root-relative path to be used for dbt-pumpkin-path conf property.
The actual limitation is less strict: source YAML path should be idempotent. For that it's enough to validate that target YAML path is sibling to the original YAML path.

Example:

# sources.yml
version: 2
sources:
  - name: my_source
    tables: [ ... ]
  - name: my_another_source
    tables: [ ... ]

# dbt_project.yml
...
sources:
  +dbt-pumpking-path: "_source_{name}.yml"

Expected files:

# _source_my_source.yml
version: 2
sources:
  - name: my_source
    tables: [ ... ]

# _source_my_another_source.yml
version: 2
sources:
  - name: my_another_source
    tables: [ ... ]

This allows to make sure that target file path doesn't depend on previous invocations and is definitely defined.