Automatically import SDWebImage when user write import SDWebImageSwiftUI

[dreampiggy]

This close #99

[dreampiggy]

We already do this for CocoaPods && Carthage user, beucase we add this in SDWebImageSwiftUI.h: https://github.com/SDWebImage/SDWebImageSwiftUI/blob/master/SDWebImageSwiftUI/Module/SDWebImageSwiftUI.h

However, SwiftPM seems suck and can not contains both Swift && Objc code in same target. So I don't add SDWebImageSwiftUI.h into the compile unit.

[codecov]

Codecov Report

Merging #100 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #100   +/-   ##
=======================================
  Coverage   75.53%   75.53%           
=======================================
  Files           9        9           
  Lines         748      748           
=======================================
  Hits          565      565           
  Misses        183      183           

Flag	Coverage Δ
#ios	75.53% <ø> (ø)

Impacted Files	Coverage Δ
SDWebImageSwiftUI/Classes/SDWebImageSwiftUI.swift	100.00% <ø> (ø)

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