The conscientious comedian Steve Coogan has gained what little attention he’s found in this country thanks to his reflexive, chatty roles in Michael Winterbottom’s “24 Hour Party People” and “Tristram Shandy: A Cock and Bull Story,” and maybe, to stateside fans of British television, for his full-bodied portrayal of the deliciously repulsive Alan Partridge. But even fans may not fully recognize him in his new role. “Hamlet 2,” which opens nationally on Friday, finds the actor shorn of his accent and knee-deep in our current brand of homegrown comic manure — namely weak, semi-ironic parody, packed with “South Park”-style explosions of vulgarity, abuse, and self-consciously awful musical numbers.