Make CommonCrypto dependency work for both Swift <4.2 and Swift 4.2 (CommonCrypto is public)

[ziogaschr]

In Swift 4.2 and Xcode 10.0 the CommonCrypto framework became publicly available in iOS. For this reason, we had to remove the modulemap. For doing this, while keeping backward compatibility with later versions, we used an elegant method from @radianttap, who used a Build phase to check for CommonCrypto existence.

p.s.: Thanks @radianttap. I hope you are fine that I used your elegant handler.

[codecov]

Codecov Report

Merging #190 into develop will decrease coverage by 0.14%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #190      +/-   ##
===========================================
- Coverage    97.22%   97.08%   -0.15%     
===========================================
  Files            6        6              
  Lines          397      377      -20     
===========================================
- Hits           386      366      -20     
  Misses          11       11

Impacted Files	Coverage Δ
Sources/Generator.swift	100% <0%> (ø)	⬆️
Sources/Token.swift	100% <0%> (ø)	⬆️
Sources/PersistentToken.swift	100% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 09b940e...47cacac. Read the comment docs.

[mattrubin]

Thanks for the PR, @ziogaschr! The changes were failing to build on Xcode 9.x, so I made a small change and pushed it to your branch to get the CI builds passing.

I like this new method of generating a modulemap at compile time only if necessary, rather than maintaining and linking external modulemap files. Before merging this PR, I'm going to test this new approach with Carthage and CocoaPods to make sure it works for all use cases.

[codecov]

Codecov Report

Merging #190 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #190   +/-   ##
========================================
  Coverage    97.22%   97.22%           
========================================
  Files            6        6           
  Lines          397      397           
========================================
  Hits           386      386           
  Misses          11       11

Impacted Files	Coverage Δ
Sources/Crypto.swift	100% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 09b940e...47cacac. Read the comment docs.

[ziogaschr]

Oh nice catch @mattrubin, I tried to see if the patch works in another 3rd party lib and it worked nicely in Xcode 9, so I thought of just copy it here without testing it below Xcode10. Nice lesson for me, to not trust any code, likely Travis catch it.

[mattrubin]

I was never quite able to get this new approach working when building with CocoaPods, and now I've just updated the whole project to build with Xcode 10, removing the need for the modulemap shims. Thank you to @ziogaschr for the PR!

[ziogaschr]

I am glad there is a better way of doing this @mattrubin. Thx for maintaining this lib.