Enforce that user defined fields/swarms/etc. don't include :: in their names

[lroberts36]

Currently, we reserve internal_varname_separator = "::" for places where parthenon internally adds stuff to field names (e.g. "bnd_flux::" + other_name and when variables are private to a package). Right now, we don't actually check that user defined variable names don't include ::. It would be good to have this check, but it is a little challenging to add because of the way package resolution works, as fields get added by users and internally using the same functions.