watchOS SPM support

[jostster]

Feature proposal

• Firebase Component: Crashlytics

Figured I would create a central place to talk about watchOS SPM support. One option is using the platform param for the SPM file which will allow some of the modules that support watchOS to be built for that platform.

[jostster]

https://docs.swift.org/package-manager/PackageDescription/PackageDescription.html#target-dependency is in Swift5.3, might be helpful.

[paulb777]

Based on https://forums.swift.org/t/platform-specific-products/39186, looking for ideas about an approach to define watchOS support for some of the Firebase products without having to ifdef out all the rest for watchOS.

[jostster]

Would something like this work? https://forums.swift.org/t/package-manager-conditional-target-dependencies/31306/2?

[paulb777]

Thanks @jostster ! That approach is looking promising. I'll try to get a PR up in the next day or two ...