Refactor populateInverseRelation() to separate ArrayAccess implementation

[Tigrov]

Q	A
Is bugfix?	✔️/❌
New feature?	❌
Breaks BC?	❌
Fixed issues	#308

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 86.72%. Comparing base (d8fa21d) to head (238b1c4).

Files	Patch %	Lines
src/ActiveRelationTrait.php	72.72%	12 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #354      +/-   ##
============================================
- Coverage     87.05%   86.72%   -0.33%     
- Complexity      599      600       +1     
============================================
  Files            13       13              
  Lines          1390     1401      +11     
============================================
+ Hits           1210     1215       +5     
- Misses          180      186       +6     

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