Prevent too aggressive reuse of interfaces used for Plexus/Sisu

[kwin]

components

Otherwise org.apache.maven.doxia.parser.module.ParserModules/ org.apache.maven.doxia.macro.Macros may be registered twice (through both m-site-p and reporting-p classloaders)

This closes #141

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[kwin]

Compare with apache/maven-site-plugin#1269 (comment) and eclipse-sisu/sisu-project#253.