CIRCLE-12024: Collapse looks for yaml files that start with "@" for specialCase

[zzak]

[CIRCLE-12024]

Use cases

1. Local jobs

.circleci/
  config.yml
  jobs/
    foo.yml

With the contents of the job being:

# .circleci/jobs/foo.yml
description: my job

2. Local jobs inline in a single-file

.circleci/
  config.yml
  @jobs.yml

Because we fold these type of files under the key of the folder their in, a jobs files would need the top-level to declare jobs:.

# .circleci/@jobs.yml
jobs:
  foo:
    description: my job

3. Inline Local jobs in an orb

This could even work for orbs.

.circleci/
  orbs/
    myorb/
      @orb.yml
      @jobs.yml

Allowing flexibility for smaller orbs that don't need nested folders.

# .circleci/orbs/myorb/@jobs.yml:
jobs:
  myjob:
    description: omg a job