[native_toolchain_c] Export tools in package

I was playing around with native assets (at https://github.com/miDeb/turbo_jpeg_ffi/tree/v3_all-platforms for reference), and I found I needed the location of the Android NDK and of vcvars64 on Windows. These can be imported
```dart
import 'package:native_toolchain_c/src/native_toolchain/android_ndk.dart';
import 'package:native_toolchain_c/src/native_toolchain/msvc.dart';
```
but they are not exported in `native_toolchain_c.dart` so you can't `import 'package:native_toolchain_c/native_toolchain_c.dart';` and the analyzer is unable to suggest the necessary import.

I think it would be nice if the exports in native_toolchain_c.dart could be expanded to also include those platform-specific tools. As a sidenote, I don't really understand why `resolve()` returns a List, I would expect only a single result? Or is it possible that a tool is installed multiple times on a machine?