Import FoundationEssentials instead of Foundation when available

[Cyberbeni]

Removing Foundation dependency on non-Apple platforms allows us to significantly reduce the size of the build product when using the Swift Static Linux SDK.

Hello world docker image depending on mqtt-nio went from 169MB to 85MB.

Related PR in one of the dependencies: apple/swift-nio-transport-services#209

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.12%. Comparing base (e02e237) to head (e826c91).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/MQTTNIO/MQTTPacket.swift	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #159      +/-   ##
==========================================
- Coverage   79.07%   78.12%   -0.96%     
==========================================
  Files          22       22              
  Lines        2518     1979     -539     
==========================================
- Hits         1991     1546     -445     
+ Misses        527      433      -94     

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[adam-fowler]

Can you fix the validation issue here. Ideally you'd just run swiftformat but I haven't updated the version of swiftformat in a while so you might get a load of unrelated changes.

Instead just reorder the FoundationEssentials and Dispatch imports

[adam-fowler]

I haven't had a chance to investigate how using Foundation vs FoundationEssentials affects a library but this looks good.

[Cyberbeni]

This video gives a good explanation: https://youtu.be/wn6C_XEv1Mo

[Cyberbeni]

@adam-fowler If you want I can open a PR that adds SwiftFormat to Package.swift and updates validate.sh to use it as an SPM plugin for formatting locally and install the version from Package.resolved with caching (so repeated runs are faster) in the GitHub workflow.

The only difference would be that Package.swift (and other swift files not in Sources/Tests) can't be formatted when the formatter is an SPM plugin.

mint uses an unsafe method for running scripts, so I usually recommend against using it: yonaskolb/Mint#227