Add support for `when: never` on conditional includes

What does this MR do and why?

This MR adds support for when: never on conditional includes.

The file is not included into the pipeline if:

• If no rules match.
• If a rule matches and has when: never

If there are no rules, the file is included as expected. Also note that when: null behaves the same as not specifying when:.

Feature flag: ci_support_include_rules_when_never

Implementation:

• Step 1: This MR
• Step 2: Add support for `when: always` on conditional i... (!123443 - merged)

Resolves #348146 (closed).

How to set up and validate locally

1. Add two config files to your project root:

config1.yml:

job1:
  script: echo

config2.yml:

job2:
  script: echo

2. Prior to turning on the Feature Flag, first observe that the when: value does not have any effect. Update your .gitlab-ci.yml file with the following content and run the pipeline.

include:
  - local: config1.yml
    rules:
      - if: $VAR == null
        when: never
  - local: config2.yml
    rules:
      - if: $VAR == null
        when: never

job0:
  script: echo

All 3 jobs are included in the pipeline.

Screenshot

3. Turn on the Feature Flag: Feature.enable(:ci_support_include_rules_when_never).
4. Re-run the pipeline and observe that only the base job0 is present.

Screenshot

5. Test the following config. Observe that the rule precedence is followed (first matching rule takes effect) and the when: value works as expected. Only job0 and job2 should be present in the pipeline.

include:
  - local: config1.yml
    rules:
      - if: $VAR == null
        when: never
      - if: $VAR == null
  - local: config2.yml
    rules:
      - if: $VAR != null
        when: never
      - exists:
        - non-existing.yml
      - if: $VAR == null

job0:
  script: echo

Screenshot

Also note that when: works with the exists condition as well. Example:

include:
  - local: config1.yml
    rules:
      - exists:
          - config1.yml
        when: never
      - if: $VAR == null
  - local: config2.yml
    rules:
      - if: $VAR != null
        when: never
      - exists:
          - non-existing.yml
      - exists:
          - config1.yml

job0:
  script: echo

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #348146 (closed)