refactor: NewWorkflowPlanner (#648)

feat: add flag `--no-recurse` to disable recursion when reading workflows from directories
feat: added more tests to `TestPlanner`, renamed `TestJobFileInfo` to more appropriate name `WorkflowPlanTest`
style: changed error message to lowercase, added single quotes for better visibility

Co-authored-by: Casey Lee <cplee@nektos.com>

pkg/model/testdata/nested/success.yml

@@ -0,0 +1,9 @@
+name: Hello World Workflow
+on: push
+
+jobs:
+  hello-world:
+    name: Hello World Job
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo "Hello World!"