Issue Description:

A lot of the SwiftPM unit tests use the ToolsSupportCore method `writeFileContents(_ path: AbsolutePath, body: (WritableByteStream) -> Void) throws`, which conveniently creates the parent directory of the file if needed.

But the core funnel method `func writeFileContents(_ path: AbsolutePath, bytes: ByteString)` does not have a way of creating the directory.

It would be useful to be able to create the directory from any of the variants of `func writeFileContents(_ path: AbsolutePath, bytes: ByteString)`, so this SR suggest adding a parameter to do that. It probably needs to default to false for compatibility, but that can be discussed.