Plugin registry compares plugins by equality, not identity #760

🤖 Written by Claude Opus 5.5 via Claude Code for the pluggy maintainers; I prompted it, it did the work, I read it.

register() (plugin in self._name2plugin.values()), get_name() and is_registered() use ==, while get_hookcallers() uses is. Two distinct plugins that compare equal are treated as one:

@dataclass
class P:
    @hookimpl
    def myhook(self): ...

pm.register(P(), "one")
pm.register(P(), "two")  # ValueError: Plugin already registered under a different name

Proposal: use identity everywhere. Small behavior change for plugins with a custom __eq__.

Found while working on #759.