fix(core): system-independent relative path sorting for FILES output

[joscha]

Fixes #19616 (comment)

The problem is that in the code previously, sorting was done on the platform-specific paths with the platform-specific separator. \ and / sort in different places, hence the order changes.

This pull request does not sort on the file objects, but on the relative, platform-independent path. I wasn't able to use org.apache.commons.io.comparator.PathFileComparator directly, as it expects a File object, but thankfully the implementation is trivial and we have lambdas :)

Tested on Windows here: #19616 (comment)

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cc @macjohnny

[joscha]

sort the relative path strings, after they've been made platform-independent in

openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultGenerator.java

Line 1994 in 849b3e8

relativePath = relativePath.replace(File.separator, "/");

[wing328]

looks good to me

thanks for the PR

fyi @OpenAPITools/generator-core-team