is_plugin_active is unreliable

[jdevalk]

Using is_plugin_active() (or the completely unnecessary edac_check_plugin_active function) is completely unreliable. Some people rename the plugin folders, others use symlinks, etc. Instead of using is_plugin_active() we should check if a plugin is active using other methods. Most plugins define constants, classes or functions that rarely (if ever) change name. Using defined(), class_exists() or function_exists() is a lot more reliable than is_plugin_active().