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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