RepositoryTransformersExtension does not handle plugin repositories

[snicoll]

#42333 has introduced a way to abstract repositories. d44e7c9 in particular is replacing the repositories and pluginRepositories section of the integration tests. But plugins have a different structure so this fails as follows:

[WARNING] Some problems were encountered while building the effective settings |  
-- | --
  | [WARNING] Unrecognised tag: 'repository' (position: START_TAG seen ...</pluginRepository>\n\t\t\t\t<repository>... @53:17)  @ /tmp/maven-build14127127292523585751/settings.xml, line 53, column 17 |  
  | [WARNING]