Use relative paths when accessing file nodes

[uhafner]

Fixes #610.

Downstream of jenkinsci/coverage-model#8

TODO:

• Use the resolved paths in the painter and remove duplicate code

[codecov]

Codecov Report

Merging #627 (3fd4d09) into master (4adf39a) will increase coverage by 0.05%.
The diff coverage is 72.66%.

@@             Coverage Diff              @@
##             master     #627      +/-   ##
============================================
+ Coverage     72.01%   72.07%   +0.05%     
- Complexity     1596     1603       +7     
============================================
  Files           127      128       +1     
  Lines          6153     6227      +74     
  Branches        654      664      +10     
============================================
+ Hits           4431     4488      +57     
- Misses         1505     1519      +14     
- Partials        217      220       +3     

Impacted Files	Coverage Δ
...ugins/coverage/metrics/source/SourceViewModel.java	0.00% <0.00%> (ø)
...gins/coverage/metrics/steps/ChangesTableModel.java	13.04% <0.00%> (ø)
...s/plugins/coverage/metrics/steps/CoverageStep.java	69.52% <ø> (ø)
...ins/coverage/metrics/steps/CoverageTableModel.java	7.97% <0.00%> (ø)
...s/plugins/coverage/metrics/steps/PathResolver.java	65.47% <65.47%> (ø)
...gins/coverage/metrics/steps/CoverageViewModel.java	54.19% <66.66%> (+0.41%)	⬆️
...ins/coverage/metrics/source/SourceCodePainter.java	61.20% <71.42%> (+1.76%)	⬆️
.../coverage/metrics/charts/TreeMapNodeConverter.java	92.85% <100.00%> (+0.96%)	⬆️
...overage/metrics/steps/CoverageChecksPublisher.java	82.55% <100.00%> (-0.86%)	⬇️
...ugins/coverage/metrics/steps/CoverageReporter.java	85.82% <100.00%> (+1.61%)	⬆️
... and 1 more

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