fix(publish): report publishConfig.tag in publish and stage notices

[ljharb]

A package's publishConfig can set tag, eg "publishConfig": { "tag": "next" }. npm publishes with it, since publishConfig is flattened into opts and the publish uses opts.defaultTag, but the "Publishing to … with tag …" notice, and the npm stage "has been staged with tag …" notice, printed defaultTag, which is read from the CLI config before publishConfig is applied. So npm publish said "with tag latest" while publishing to next.

Both notices now print opts.defaultTag, the tag actually used, with tests for npm publish and npm stage.