In case when `needs:` is missing, but when requested by service,
we would not save the pipeline with config_error.

This makes it explicit that we want to persist the error
as `config_error` failure reason.

This changes the `needs:` logic to require
that all jobs to be present. Instead of skipping
do fail the pipeline creation if `needs:` dependency
is not found.

This removes the Protect pipeline chain step and assigns the protected
attribute in the Populate step instead.

Enables frozen string for the following:

* lib/gitlab/ci/*.rb
* lib/gitlab/ci/build/**/*.rb
* lib/gitlab/ci/config/**/*.rb
* lib/gitlab/ci/pipeline/**/*.rb
* lib/gitlab/ci/reports/**/*.rb

Partially addresses #47424.

This is a first step to decouple pipeline from YAML processing. It
reduces the coupling by removing some methods that introduce coupling
and by moving logic into separate chain element that is being used to
populate pipelines with stages and builds.