Improve imports and deep-merging of variables of any complex types

[aknysh]

what

• Improve imports and deep-merging of variables of any complex types
• Use the latest variant2 features (recursive user-defined functions, list of file paths in config) for stack configuration module

why

• Simplify code
• Deep-merge any number of import files with any complex structures (maps/objects)
• Deep-merge all variables in this order: global-scoped, component-type-scoped, and component-scoped
• Deep-merge all backend attributes in this order: global-scoped, component-type-scoped, and component-scoped

references

• Support for merging multiple config files from variable mumoshu/variant2#43

test

 ✗ . example ⨠ atmos stack config eks -s ue2-dev --config-type=vars
"cluster_kubernetes_version": "1.18"
"environment": "ue2"
"namespace": "eg"
"region": "us-east-2"
"stage": "dev"
"test-map":
  "a": "a4"
  "atr":
    "atr1": "1-1"
    "atr2": "2-4"
    "atr3": 3
    "atr4": 4
    "atr5": 5
    "list":
    - "1c"
  "atr-2":
    "atr1": 1
    "atr2": 2
  "b": "b2"
  "c": "c1"
  "d": "d4"
  "e": "e4"
  "f": "f4"
  "g": "g1"
  "list":
  - 4
  - 5
  - 6
  "list2":
  - 1
  - 2
  - 3