Handle @latest as `dist-tag`

At the moment, only `@latest` is considered as being a valid `dist-tag`.

Although it is pretty handy to do a [`npm publish --tag blah`](https://www.npmjs.org/doc/cli/npm-publish.html) to then `npm install package@blah` to alias a version and keep it moving.

Any plan to support that explicitly match the `npm` tag thing?

Thanks :-)

No files, tests, or entry points are named. Start by locating the code that validates package dist-tags, compare its behavior with npm's arbitrary publish tags such as `blah`, and define done as resolving packages installed with those tags; add or update coverage if the repository has tests.

Written by the indexing model from the issue text.