feat(spm): Add method to determine if a plugin is a Swift package

[dpogue]

Platforms affected

iOS

Motivation and Context

We want a declarative way for a plugin to opt-in to using Package.swift for managing its code and resources as opposed to file copying with source-file/header-file/resource-file/etc.

Description

Adds a helper method to detect if there is a <platform name="ios" package="swift"> tag.

Caveats:

• This currently only looks at the first iOS platform declaration
• This doesn't currently check the value of the package attribute. If it exists, we assume it is a Swift package. Ideally I'd love for this to be a boolean attribute like in HTML, but XML doesn't support those 😒

Testing

Added unit test case.

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.42%. Comparing base (24f7fa2) to head (5005e3f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
+ Coverage   88.40%   88.42%   +0.02%     
==========================================
  Files          20       20              
  Lines        1147     1149       +2     
==========================================
+ Hits         1014     1016       +2     
  Misses        133      133              

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[dpogue]

I found a way to handle this in the platform that doesn't involve modifying cordova-common, so I'm going to close this