Error out on `--target` if specified directory is non-empty unless `--force-target` is used

[notatallshaw]

No one seems to understand what --target should do in a non-empty directory and users seem to have assumptions about what it should do, even though lots of warnings are thrown, e.g.

WARNING: Target directory /tmp/z/zz/out/cffi already exists. Specify --upgrade to force replacement.

I believe a better UX would be to error out with a message like:

ERROR: /tmp/z/zz/out is non-empty and the behavior or `--target` is undefined, use `--force-target` to override.

Would this cause too much churn? And is so what are users doing with --target now? Would a deprecation period allow us to find real world use cases and see if there is some behavior we are willing to support?