fix(core): fix populating relations in parallel via Promise.all

[B4nan]

This reworks the fix for #4213 which broke populating in parallel.

Closes #4343

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (ef14581) 99.61% compared to head (dde9411) 99.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4415      +/-   ##
==========================================
- Coverage   99.61%   99.61%   -0.01%     
==========================================
  Files         214      214              
  Lines       14177    14176       -1     
  Branches     3354     3354              
==========================================
- Hits        14122    14121       -1     
  Misses         54       54              
  Partials        1        1              

Impacted Files	Coverage Δ
packages/core/src/entity/WrappedEntity.ts	100.00% <ø> (ø)
packages/core/src/typings.ts	100.00% <ø> (ø)
packages/core/src/entity/EntityLoader.ts	100.00% <100.00%> (ø)

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