fix framework imports with private swiftinterface

[adincebic]

Imported frameworks/XCFrameworks can ship both a primary .swiftinterface file and a companion .private.swiftinterface file for the selected target triplet. We were only propagating a single Swift interface input, which could cause Swift
compilation to miss the private interface when importing the framework.

To fix it:

• I update target-triplet Swift module selection to collect both regular and .private interface files
• make sure to propagate the full set of Swift interface inputs through framework/XCFramework import support instead of only the first interface

We didn't see this fail since when building with rules_xcodeproj sandbox is disabled thus those files were getting discovered. Enabling sandboxed or worker strategy for SwiftCompile and SwiftDeriveFiles mnemonic surfaces the issue.