Pre-aggregations are not inherited via `extends` in YAML-based models

[igorlukanin]

Describe the bug
Pre-aggregations are not inherited via extends in YAML-based models.
However, they do in JS-based ones.

To Reproduce
Steps to reproduce the behavior:

1. Use provided data model
2. Check pre-aggregations
3. See that there are only 3 pre-aggregations

Expected behavior
4 pre-aggregations, two in base cubes, two in child cubes.

Minimally reproducible data model

cubes:
  - name: pa_yml
    sql: SELECT 132

    measures:
      - name: count
        type: count

    pre_aggregations:
      - name: main
        measures:
          - count
  
  - name: pa_yml_ext
    extends: pa_yml

cube(`pa_js`, {
  sql: `SELECT 132`,

  measures: {
    count: {
      type: `count`
    }
  },

  pre_aggregations: {
    main: {
      measures: [
        count
      ]
    }
  }
})

cube(`pa_js_ext`, {
  extends: pa_js
})

Version:
v1.2.0+