Move DAP breakpoint event code to breakpoint.py

Move DAP breakpoint event code to breakpoint.py

A subsequent patch will add the ability to suppress breakpoint events
to DAP.  My first attempt at this ended up with recurse imports,
causing Python failures.  So, this patch moves all the DAP breakpoint
event code to breakpoint.py in preparation for the change.

I've renamed breakpoint_descriptor here as well, because it can now be
private to breakpoint.py.