Deprecate hierarchical config

[arturbosch]

Needs rebase on top of #2420. Please review just the last commit.

A Config is a long lived object and is derived via subConfig a lot.
Keeping track of the parent it was derived, creates long-lived object chains which takes the GC longer to release them.
It can even lead to OOM if detekt get's embedded in an other application which reuses the top most Config object.
The property 'parentPath' of the Config interface can be used as a replacement for parent.key calls.
I consider the introduction of HierarchicalConfig.Parent as a bug and will remove it after two minor versions.

[codecov-io]

Codecov Report

Merging #2421 into master will decrease coverage by 0.01%.
The diff coverage is 88.88%.

@@             Coverage Diff              @@
##             master    #2421      +/-   ##
============================================
- Coverage     82.85%   82.83%   -0.02%     
+ Complexity     2174     2166       -8     
============================================
  Files           356      356              
  Lines          6137     6136       -1     
  Branches       1121     1119       -2     
============================================
- Hits           5085     5083       -2     
- Misses          476      478       +2     
+ Partials        576      575       -1

Impacted Files	Coverage Δ	Complexity Δ
...tlab/arturbosch/detekt/api/internal/EmptyConfig.kt	66.66% <ø> (-16.67%)	4 <0> (-1)
...ain/kotlin/io/gitlab/arturbosch/detekt/api/Rule.kt	94.11% <100%> (ø)	13 <1> (-4)	⬇️
...n/kotlin/io/gitlab/arturbosch/detekt/api/Config.kt	94.11% <100%> (+0.36%)	0 <0> (ø)	⬇️
...itlab/arturbosch/detekt/api/internal/BaseConfig.kt	86.36% <100%> (+0.64%)	10 <2> (-2)	⬇️
...itlab/arturbosch/detekt/api/internal/YamlConfig.kt	76.66% <83.33%> (-0.26%)	9 <1> (+1)

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