gdb/dap: use breakpoint fullname to resolve source

gdb/dap: use breakpoint fullname to resolve source

If the breakpoint has a fullname, use that as the source path when
resolving the breakpoint source information. This is consistent with
other callers of make_source which also use "fullname" if it exists (see
e.g. DAPFrameDecorator which returns the symtab's fullname).

Approved-By: Tom Tromey <tom@tromey.com>