Support `use_frameworks` in Swift example

[louiszawadzki]

Hi!

The Swift example as it is does not support apps using use_frameworks! in their Podfile.
When using frameworks, the Swift import is different.

Instead of:

#import "ProductModuleName-Swift.h"

It should be:

#import <ProductName/ProductModuleName-Swift.h>

So users with use_frameworks! will get an error at build stating that:

fatal error: 'ProductModuleName-Swift.h' file not found

To handle both cases from inside the library, we can use #if __has_include as recommended on this Cocoapods issue.
I'd recommend changing the import in calculator/ios/RNCalculator.mm to be (not tested):

#if __has_include("calculator-Swift.h")
#import <calculator-Swift.h>
#else
#import <calculator/calculator-Swift.h>
#endif

For reference, here is the link to the change we ended up making.