Detect dead paths in YAML configuration

[fridex]

For the following configuration:

tasks:
  - name: A
    import: foo.bar

  - name: B
    import: foo.bar

  - name: C
    import: foo.bar

  - name: D
    import: foo.bar

flows:
  - flow1

flow-definitions:
    - name: flow1
      edges:
        - from:
          to: A
        - from: A
          to: B
        - from:
            - B
            - C
          to: D
        - from: D
          to: C

selinonlib should report error as C and D will never be run due to cyclic dependencies. We should examine attainability of graph nodes in YAML config file.