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.
