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When installing htmlpurifier.module using an install profile (and possibly other installation use cases), with the required htmlpurifier library installed in sites/all/libraries - htmlpurifier_requirements() returns severity=REQUIREMENT_ERROR and won't continue (wanting the library to be installed in sites/default/module/htmlpurifier/library). I have libraries.module installed (but due to being in installation profile it would not yet be enabled). This is probably because the libraries.module is not a dependency (for legacy reasons).
The attached patch fixes this particular use case by checking for the libraries.module and including it.