Fix diagram images path in site modules

[abelsromero]

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What is this issue about?

• Bug report
• Feature request
• Question

Description

Using asciidoctorj-diagram with the maven-site modules causes diagram images to be created in the source path.

Environment information

• asciidoctor-maven-plugin version: 2.2.5
• asciidoctorj version: 2.5.11
• asciidoctorj-diagram version: 2.2.17
• Maven, Java and OS version: at least Windows and Linux Example with maven site plugin and diagrams asciidoctor-maven-examples#238 https://asciidoctor.zulipchat.com/#narrow/stream/279642-users/topic/Use.20PlantUML.20in.20Maven.20Site

[abelsromero]

The issue is caused because the site converter converts to String and then Doxia (Maven's site component) creates the image files from it. Hence asciidoctor-diagram generates images in the only path available, the one where the source is.

The current workaround of setting imagesoutdir attribute won't work when the sources are found in different sub-folders.