Consider the following .gitlab-ci.yml:

```
stages:
    - test
    - build
    - deploy

test:
    stage: test
    script:
        - echo 1
build:
    stage: build
    script:
        - echo 1

deploy:
    stage: deploy
    needs: ["build"]
    dependencies: "test" # notice: no brackets
    script:
        - echo 1
```

If either `needs` or `dependencies` were not arrays of strings, the
validation for missing needs would fail because it was trying to
calculate `dependencies - needs`, which is invalid.

Even though `dependencies` and `needs` have separate validations that
check their types, the validation chain isn't halted if an error is
encountered. Thus, the validation for missing needs would still try to
run.

We fix this by checking the normalized (aka composed) job values in the
needs section.

Closes https://gitlab.com/gitlab-org/gitlab/issues/195653