[MSITE-901] If preceding standalone report has been run, site:jar do…

[michael-o]

…es not reinvoke site:site

This closes #82

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[michael-o]

Will be back ported to 3.x as well: https://github.com/apache/maven-site-plugin/tree/MSITE-901-3.x

@mattnelson This one is for you!

[jira-importer]

Resolve #1042