Fix: Make RCTransaction public

[aboedo]

Addresses #314

The issue was Transaction was forward-declared, but its implementation wasn't publicly available, so calling purchaserInfo.nonSubscriptionTransactions worked, but trying to access properties of the transaction wouldn't work.

This is the first public Swift file, so figuring out the best path to make it public was sort of tricky, mostly because of the difference in structure we have between most package managers and SPM.
For most package managers, it's only a matter of declaring the type public.

However, since SPM isn't compatible with mixed-language frameworks, we separated its code into an objc and a swift target, with the objc target being the public-facing one, which depends on swift.
The issue there is that a public swift file wouldn't be publicly available.

I tried a few approaches:

• Keeping the file internal in swift and then providing a public interface for it is a problem for package managers other than SPM because it effectively generates a duplicated symbol. This doesn't happen if both swift and obj parts are internal, but it's a problem if either of the two is public.
• I tried re-structuring the SPM solution so that a 3rd framework that unifies both parts is the public-facing one, and then using @_exported import PurchasesObjc and @_exported import PurchasesSwift to expose the public stuff in both. That almost worked, but the objc portion won't build for some reason that escapes me.
• I tried having a header that uses a pre-processor macro so that it's only included in SPM. That didn't work because the pre-processor macros don't get included when used for public stuff, only internal. (See 'HEADER TRAPS' here).

The current approach is to have:

• the file be public for all package managers.
• a public objc interface for the file, that's only used by SPM. In order to achive this bit, there's a new folder in Public called SPMSwiftInterfaces, for this purpose. It's automatically included in SPM because it's located in Public.
• exclude Public/SPMSwiftInterfaces in the podspec
• exclude the files in that folder from xcode by not including them in the target.

I think at some point we may want to restructure the project so that there's no difference between SPM and other package managers.

[aboedo]

one easy way to generate the code for this file, so that you don't have to manually do the conversion into objc, is to make the file public in swift, then go to derived data and look for Purchases-swift.h. That file is an objc version of the public interface for the framework, you just have to find the class and copy/paste it. then maybe clean it up a bit.