Move (most of) update_module_enable() into UpdateModuleHandler::enable()

Spin off from #2001310: Disallow firing hooks during update. That issue is making the update system more robust and testable, but has some tricky code in it that could use more thorough review. To help focus that work, this issue just extracts the part of that one that moves most of the update_module_enable() implementation into a new class: UpdateModuleHandler.

This patch is almost a direct copy/paste of update_module_enable() to UpdateModuleHandler::enable(), except it makes UpdateModuleHandler::enable() conform to ModuleHandlerInterface::enable(), which means there's a little wrapping code retained in update_module_enable() to be cleaned up in the other issue.

Please credit chx on commit, since he wrote the initial patch from which this is extracted.