Failed to validate extension that depends on com.atlassian.sal:sal-api

What I did in master feels a bit too dangerous for the LTS.

Now we could simply add a hard-coded "atlassian-plugin" to https://github.com/xwiki/xwiki-commons/blob/stable-15.10.x/xwiki-commons-core/xwiki-commons-extension/xwiki-commons-extension-maven/src/main/java/org/xwiki/extension/maven/internal/MavenUtils.java#L88 and https://github.com/xwiki/xwiki-commons/blob/stable-15.10.x/xwiki-commons-core/xwiki-commons-extension/xwiki-commons-extension-repositories/xwiki-commons-extension-repository-maven/src/main/java/org/xwiki/extension/repository/aether/internal/XWikiRepositorySystemSession.java#L72, it should be enough to cover this specific use case.