Add support for App Extensions

[erikbdev]

This pull request adds support for Apple's App Extensions.

Closes: #45

[kabiroberai]

this is great, thanks! left one quick comment but feel free to ping me once it's ready for broad review.

[erikbdev]

@kabiroberai I am 90% done with App Extensions (excluding ExtensionKit, iMessage).

The only thing missing is ensuring XcodeProj generates a valid folder with all products.

If you like you can go over my code base and let me know if you have any feedback. :)

[kabiroberai]

@erikbdev thanks, I'll look at this as soon as I can! In the meantime do you mind fixing the SwiftLint failures? You can run make lint to lint locally.

[kabiroberai]

lgtm, this is great! will do some final testing and merge shortly if everything works.

A few things we probably want to do as followups:

1. [High priority] support ExtensionKit
2. [Medium priority] deduplicate frameworks. i.e. if a framework is used across multiple products (multiple extensions or main target + extension), pull it out into the main target's Frameworks directory and reuse it.
3. [Low priority] create an upstream SwiftPM issue about describe and show-dependencies having logs that are sent to stdout. I think the ideal fix here would be for describe to offer a -o parameter because that entirely skirts around the issue of noisy stdout logs.

[kabiroberai]

Another idea for a followup: I wonder whether a shorthand to avoid requiring extension.[].infoPlist would be useful. For example,

extensions:
- product: MyWidget
  NSExtensionPointIdentifier: com.apple.widgetkit-extension
- product: MyGenericExt
  EXExtensionPointIdentifier: com.example.generic

or maybe

extensions:
- product: MyWidget
  extensionPoint: com.apple.widgetkit-extension
- product: MyGenericExt
  type: ExtensionKit
  extensionPoint: com.example.generic

[erikbdev]

extensions:
- product: MyWidget
  extensionPoint: com.apple.widgetkit-extension
- product: MyGenericExt
  type: ExtensionKit
  extensionPoint: com.example.generic

This looks interesting, we could also possibly infer the type of extension based on the extensionPoint value. One thing to note is that some extensions require additional keys, e.g. "NSExtensionPrincipalClass" or "NSExtensionMainStoryboard."

[kabiroberai]

we could also possibly infer the type of extension based on the extensionPoint value

hmm we could but that feels a bit too magical to me. I think being explicit about it (either via type: ExtensionKit or the NSExtension/EXAppExtensionAttributes key) is ideal.

One thing to note is that some extensions require additional keys, e.g. "NSExtensionPrincipalClass" or "NSExtensionMainStoryboard."

I think we should just require an Info.plist in these cases. As newly introduced extension points move towards ExtensionKit, this will hopefully become less necessary over time.