When GDB has just started, several `gdb` submodules are automatically
loaded, so user code does not have to manually import them (for instance
`import gdb.events`). Reflect that in `gdb` stubs.
* gdb.events.BreakpointEvent: fix various typos
* gdb.printing: remove GenPrinterFunction
This was meant to be a private type alias, but it is unused and a
duplicate of the better named gdb._PrettyPrinterLookupFunction.
