Custom XInclude attributes and resolution mechanisms

[novotnyr]

• Support includeIf and includeUnless attributes in the XInclude elements. Properties are resolved against System properties. This feature requires com.jetbrains.plugin.structure.intellij.xinclude.isResolvingConditionalIncludes system property to be set to true. By default, an include element with either of these attributes will be ignored and not processed for inclusion.

  This feature is implemented by the Kotlin plugin and should not be used in other plugins.

• Resolve issues with XMLs referencing each other and being set in META-INF and resource roots.
  This is an issue in Kotlin plugin. Imagine the following ZIP file contents

|- META-INF
     | - plugin.xml
     | - kotlin-core.xml
|- kotlin.plugin.k1.xml

The plugin.xml conditionally includes kotlin.plugin.k1.xml (via href="kotlin.plugin.k1.xml").
Additionally, kotlin.plugin.k1.xml includes kotlin-core.xml, again via href="kotlin-core.xml".

This is not in line with XInclude specification. However, the support for such scenarios is implemented.

• See also MP-6594 Could not find bundled plugin with ID: 'org.jetbrains.kotlin'

[YannCebron]

Please add a comment in relevant places that this is a special case for Kotlin plugin and not a regular feature for plugin.xml.