Merge XML report output

[chao2zhang]

This is a rewrite of #3394 and addresses one item in #3360.

• Only the gradle tasks are exposed: No opinionated way to configure the tasks.
• Documentation is added.
• Fix: utf-8 to UTF-8, because the latter is the standard.

[codecov]

Codecov Report

Merging #3491 (130ce1b) into master (67dc147) will increase coverage by 0.44%.
The diff coverage is 76.92%.

@@             Coverage Diff              @@
##             master    #3491      +/-   ##
============================================
+ Coverage     80.06%   80.51%   +0.44%     
- Complexity     2864     2893      +29     
============================================
  Files           458      461       +3     
  Lines          8584     8668      +84     
  Branches       1663     1676      +13     
============================================
+ Hits           6873     6979     +106     
+ Misses          794      767      -27     
- Partials        917      922       +5     

Impacted Files	Coverage Δ	Complexity Δ
.../main/kotlin/io/gitlab/arturbosch/detekt/Detekt.kt	27.27% <ø> (ø)	20.00 <0.00> (ø)
...lab/arturbosch/detekt/report/XmlReportMergeTask.kt	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...gitlab/arturbosch/detekt/report/XmlReportMerger.kt	90.47% <90.47%> (ø)	5.00 <5.00> (?)
...lin/io/github/detekt/report/xml/XmlOutputReport.kt	91.30% <100.00%> (ø)	7.00 <0.00> (ø)
...kt/rules/documentation/AbsentOrWrongFileLicense.kt	87.50% <0.00%> (-5.36%)	12.00% <0.00%> (+4.00%)	⬇️
.../arturbosch/detekt/internal/DetektMultiplatform.kt	90.90% <0.00%> (ø)	3.00% <0.00%> (?%)
...kotlin/io/gitlab/arturbosch/detekt/DetektPlugin.kt	86.66% <0.00%> (+5.71%)	10.00% <0.00%> (+1.00%)
...ules/documentation/LicenceHeaderLoaderExtension.kt	80.95% <0.00%> (+9.07%)	6.00% <0.00%> (ø%)
... and 2 more

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[schalkms]

I really like this feature. I had a short look at it. It works fine. Great! 👍

I think it's good to start small with the XML report format. For me XML reports supporting a merge is the first logical step. Other reports could follow later on, if this is highly valuable.

I'm not sure regarding the test coverage for the XML parsing logic and task. Should we include more edge cases?

[chao2zhang]

I really like this feature. I had a short look at it. It works fine. Great! 👍

I think it's good to start small with the XML report format. For me XML reports supporting a merge is the first logical step. Other reports could follow later on, if this is highly valuable.

I'm not sure regarding the test coverage for the XML parsing logic and task. Should we include more edge cases?

The logic is relatively straightforward so I think the existing test coverage should be fine and jacoco is happy about hitting the 80% coverage in this diff.

The overall project test coverage is increased, but the patch diff test coverage is lowered. We probably need to look into why it is giving us conflicting test coverage.