refactor: Bring back _.flatMap usage

[medikoo]

Reverts #9948 (it covered a bit more originally but other methods were refactored in a meantime)

There was no intention (yet) to get rid of _.flatMap in scope of #7747, as reliable alternative in form of [].flatMap doesn't work with Node.js v10

PR proposed to refactor it into less readable [].reduce logic, which I think reduced readability of logic.

We should keep _.flatMap and refactor it into native [].flatMap on the grounds of v3 (which will come with dropped support for Node.js v10)

/cc @JonasMatos0 @pgrzesik

[codecov]

Codecov Report

Merging #10004 (711484d) into master (40cdb4f) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #10004      +/-   ##
==========================================
- Coverage   86.17%   86.17%   -0.01%     
==========================================
  Files         330      330              
  Lines       13161    13160       -1     
==========================================
- Hits        11342    11341       -1     
  Misses       1819     1819              

Impacted Files	Coverage Δ
lib/plugins/aws/invokeLocal/index.js	68.81% <100.00%> (-0.09%)	⬇️

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