Export OHHTTPStubs from OHHTTPStubsSwift

[manicmaniac]

Checklist

• I've checked that all new and existing tests pass
• I've updated the documentation if necessary
• I've added an entry in the CHANGELOG to credit myself

Description

Export OHHTTPStubs from OHHTTPStubsSwift using @_exported so that users doesn't need to import both of them.

Motivation and Context

Previously SwiftPM users should import both OHHTTPStubs and OHHTTPStubsSwift, contrary to CocoaPods users who need to import only OHHTTPStubs.
This difference is small but an obstacle for migrating to Swift Package Manager.

1. Firstly I removed import OHHTTPStubs statement from Tests/OHHTTPStubsSwiftTests/SwiftHelperTests.swift. This change made tests fail.
2. Then add @_exported modifier to import statement, then the test passed.