Deprecate PluginManager.getPluginInfo: It doesn't do what its JavaDoc says

- It will not only return plugins with a matching class name but also with a matching filename
- I should have reviewed the callers for whether they need the filename matching and removed the functionality completely if they don't. But the tight WOT release schedule forbids me from spending too much time on this. So insteadd this commit just deprecates the function.
- Add replacement functions for querying by class name and by filename. Do not add one for querying by both - I doubt that this is needed. If someone needs it, it will be easy to add.