Fix SPM support and request to tag 1.8.3: fix for #63

[synthemesc]

My error, #63 only contains some of the work I intended to add to that PR. This branch is the full thing, if you could merge and re-tag 1.8.3. The change list is the same.

• Fix SPM support to explicitly include CountryPicker.bundle as a resource, otherwise countryCodes.json may not be included. Because of the guard let on line 188 of CountryPicker.swift, failure to load the JSON silently fails and returns an empty Set.
• Add conditionals to use Bundle.module when compiled with SPM.
• Update Podspec for 1.8.3 and regenerate lockfiles. I would request that this be tagged as 1.8.3 at the same time, so that SPM will see the recent SPM-specific commits for that version, instead of having to just follow master.