When `jQuery.noConflict` is called, the global `$` variable is not
defined. We cannot assume in the code that the variable is defined,
and instead should always make sure to import the `jquery` module
when we need jQuery.
This fixes all of the issues where we are assuming that `$` is
defined globally when we are not importing the module. This also
fixes the issues when defining the `jQuery.fn.select2.amd` modules
where we were always attaching to the `$` object instead of the
`jQuery` object.
This closes https://github.com/select2/select2/issues/2947.
This adds the `select` and `unselect` events so users can still
hook into them from their jQuery plugins.
This also modifies the core `trigger` to fire pre-events that can
be canceled for the `open`, `close`, `select`, and `unselect`
events. Users implementing these events who need to handle cases
where the event has been prevented should check `args.prevented`,
where `args` is the object that was included for arguments when
triggering the internal event.
