`--overwrite-policy` enhancements

**Describe the feature request**
`--overwrite-policy=upgrade`, which should be the new default: automatically overwrites if the package is being replaced by a newer version. (For replacing with the same or an older version, manually specifying `--overwrite-policy=always` seems reasonable.)
`--overwrite-policy=ask`, which should probably _not_ be default because of use in CI, unless it can check for `stdin` being a tty (but is this even meaningful on Windows?).

**Additional context**
Discussion on #hackage:matrix.org. https://matrix.to/#/%23hackage%3Amatrix.org/%24Y7SP4UGHQ_cN_QVGc-iJYaHwY06tdkPkxx1wIZuHdEY?via=matrix.org&via=kf8nh.com&via=mozilla.org