Steps to reproduce:

1. Create a directory called "main.ha".
2. Create a Hare file "main.ha/main.ha".
3. In "main.ha/", run hare build.
4. "main.ha/main.ha" will be overwritten with a created executable.

Go avoids this by checking whether the to-be-overwritten file is an object[1], although this might not work well with +data (#21).

[1] https://github.com/golang/go/blob/ba7f3d98d41f01291dd831e618736b02a79ef27a/src/cmd/go/internal/work/exec.go#L1917