Fix PEP cyclic dependencies error with non-standard project stages order

What does this MR do and why?

Fix PEP cyclic dependencies error with non-standard project stages order.

When project pipeline uses the default stages in a non-standard order, inject_policy pipeline execution policy has to specify the stages in the same relative order, otherwise pipelines fail to start with cyclic dependencies detected error.

This change fixes it by not collecting the policy stages unless the stages are explicitly declared.

Depends on refactoring from !194147 (merged).

References

• #545147 (comment 2635285810)
• Slack (internal)
• https://gitlab.com/gitlab-com/account-management/eastern-north-america/lululemon-athletica/lululemon-athletica/-/issues/206

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a project
2. In the project, create PEP without declaring stages:

   policy-build-job:
     stage: build
     script: exit 0
   
   policy-test-job:
     stage: test
     script: exit 0

3. Create .gitlab-ci.yml in the project which uses non-standard order of stages:

   stages: [test, build, deploy]
   
   project-build:
     stage: build
     script: exit 0
   
   project-test:
     stage: test
     script: exit 0
   
   project-deploy:
     stage: deploy
     script: exit 0

4. Run the pipeline and verify it can be created

MR acceptance checklist

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Related to #556754 (closed)