Swift Package Manager integration build failed

[pyby]

Environment

• Platform+version: Any
• FLEX version: Current master branch (1b91bc9)

Bug Report

The project support Swift Package Manager.

In an Xcode project using this Flex version with SPM, the build failed with error:
/Users/pyby/Library/Developer/Xcode/DerivedData/PlaySRG-dyyaxxrvmfadjcclckzqglcrgvfx/SourcePackages/checkouts/FLEX/Classes/Headers/FLEX-Categories.h:21:9: error build: 'NSDateFormatter+FLEX.h' file not found

Issue

Public headers has to be declare in the Package.swift file.
The publicHeadersPath is Classes/Headers folder.

One of last commit "Wrap NSDateFormatter" (b241075) added NSDateFormatter+FLEX.h in the public FLEX-Categories.h header.