[MJAVADOC-826] - Don't try to modify project source roots

[oehme]

The aggregate javadoc mojo was calling addAll directly on the list returned by project.getCompileSourceRoots. This bug was silently ignored by Maven 3, but results in an UnsupportedOperationException on Maven 4.

Do a defensive copy instead.

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[jira-importer]

Resolve #1203