Create separate SPM packages

[guykogus]

🚀

Fixes #1023

Checklist

• I checked the Contributing Guidelines before creating this request.
• New extensions are written in Swift 5.6.
• New extensions support iOS 12.0+ / tvOS 12.0+ / macOS 10.13+ / watchOS 4.0+, or use @available if not.
• I have added tests for new extensions, and they passed.
• All extensions have a clear comments explaining their functionality, all parameters and return type in English.
• All extensions are declared as public.
• I have added a changelog entry describing my changes.

[guykogus]

@LucianoPAlmeida what do you think? I was wondering if maybe we need to completely refactor it out so that each extension package exists in a separate folder. I.e. at the root of the project have SwifterSwift-AppKit/Sources/*. This seems to work, too, but it's another option.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.92%. Comparing base (39fa28c) to head (4239878).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1187      +/-   ##
==========================================
+ Coverage   93.17%   95.92%   +2.74%     
==========================================
  Files         108      108              
  Lines        4002     4002              
==========================================
+ Hits         3729     3839     +110     
+ Misses        273      163     -110     

Flag	Coverage Δ
ios	95.63% <ø> (ø)
macos	96.58% <ø> (+13.22%)	⬆️
tvos	94.27% <ø> (-1.93%)	⬇️

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[SwifterSwiftBot]

	2 Warnings
⚠️	Consider adding tests for new extensions or updating existing tests for a modified SwifterSwift extension
⚠️	The source files have been modified. Please consider adding a CHANGELOG entry if necessary.

	1 Message
📖	Thank you for submitting a pull request to SwifterSwift. The team will review your submission as soon as possible.

Generated by 🚫 Danger

[LucianoPAlmeida]

@LucianoPAlmeida what do you think? I was wondering if maybe we need to completely refactor it out so that each extension package exists in a separate folder. I.e. at the root of the project have SwifterSwift-AppKit/Sources/*. This seems to work, too, but it's another option.

I think this is fine if that works and such a refactor will be much work for no much value if the way it is serves the purpose.