Prevent import statements from counting as references for UnusedPrivateProperty

[luanpotter]

Fixes #5941

Basically ignore import statements when counting references for the purposes of identifying Unused Private Properties.

Also took the opportunity to improve the previous test package declarations are ignored a bit.

[codecov]

Codecov Report

Merging #5942 (01ca14c) into main (2187667) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #5942   +/-   ##
=========================================
  Coverage     84.48%   84.48%           
  Complexity     3789     3789           
=========================================
  Files           546      546           
  Lines         12935    12937    +2     
  Branches       2273     2275    +2     
=========================================
+ Hits          10928    10930    +2     
  Misses          877      877           
  Partials       1130     1130           

Impacted Files	Coverage Δ
...rbosch/detekt/rules/style/UnusedPrivateProperty.kt	93.54% <100.00%> (+0.21%)	⬆️

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