Handle better the case when there is a conflict between marker publications and the librarian-generated ones

Librarian.module(project) // tries to generate the markers

gratatouille {
  // creates a publication
  pluginMarker("com.apollographql.mock")
}

Later fails with:

Execution failed for task ':librarianStaticContent' (registered in build file 'build.gradle.kts').
> A failure occurred while executing com.gradleup.librarian.gradle.internal.task.GenerateStaticContentTaskWorkAction
   > /home/runner/work/apollo-mock/apollo-mock/gradle-plugin/build/gtask/librarianGenerateMarkerFiles/output/com/apollographql/mock/com.apollographql.mock.gradle.plugin/0.0.0-SNPASHOT/com.apollographql.mock.gradle.plugin-0.0.0-SNPASHOT.pom -> /home/runner/work/apollo-mock/apollo-mock/build/static/m2/com/apollographql/mock/com.apollographql.mock.gradle.plugin/0.0.0-SNPASHOT/com.apollographql.mock.gradle.plugin-0.0.0-SNPASHOT.pom: The destination file already exists.

Reproduce the conflict using the Librarian.module and pluginMarker example in the issue, then start with build.gradle.kts and the GenerateStaticContentTaskWorkAction stack-trace entry. Trace how librarian-generated marker publications are written and define the expected behavior when a marker already exists; done means this case no longer fails with a destination-file-exists error.