Add relative path for file nodes

[uhafner]

Replaces the path computation based on packages.

Fixes jenkinsci/code-coverage-api-plugin#610

Tasks:

• Optimize TreeString creation

[codecov]

Codecov Report

Merging #8 (8b5348d) into main (b39b053) will increase coverage by 0.36%.
The diff coverage is 93.10%.

@@             Coverage Diff              @@
##               main       #8      +/-   ##
============================================
+ Coverage     85.15%   85.52%   +0.36%     
+ Complexity      481      480       -1     
============================================
  Files            23       23              
  Lines          1334     1340       +6     
  Branches        195      190       -5     
============================================
+ Hits           1136     1146      +10     
+ Misses          152      148       -4     
  Partials         46       46              

Impacted Files	Coverage Δ
...in/java/edu/hm/hafner/coverage/CoverageParser.java	61.90% <71.42%> (+4.76%)	⬆️
...edu/hm/hafner/coverage/parser/CoberturaParser.java	89.07% <80.00%> (-1.11%)	⬇️
src/main/java/edu/hm/hafner/coverage/FileNode.java	91.11% <100.00%> (+0.57%)	⬆️
...c/main/java/edu/hm/hafner/coverage/ModuleNode.java	100.00% <100.00%> (ø)
src/main/java/edu/hm/hafner/coverage/Mutation.java	79.31% <100.00%> (+0.24%)	⬆️
src/main/java/edu/hm/hafner/coverage/Node.java	81.86% <100.00%> (+0.86%)	⬆️
.../main/java/edu/hm/hafner/coverage/PackageNode.java	100.00% <100.00%> (+12.00%)	⬆️
...va/edu/hm/hafner/coverage/parser/JacocoParser.java	90.13% <100.00%> (+0.26%)	⬆️
...va/edu/hm/hafner/coverage/parser/PitestParser.java	97.50% <100.00%> (ø)

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