Replaces all the foundation imports.
One issue is that `HTTPClient.init?(httpsURLWithSocketPath socketPath:
String, uri: String = "/")` uses `addingPercentEncoding()` from
Foundation. So instead, we use a pure Swift impl. that does the same.
We also need to disable default traits from `swift-configuration` to
prevent linking Foundation, because the `JSON` trait does that.
This also adds a linkage test to prevent regressions to CI.