Add epl-upgradable-packages

I think epl was missing these simple helpers... so here we go.
`find` is a `cl` function, and we don't require `cl`.

Generally, the whole function is just:

```cl
(cl-find (epl-package-name package) (epl-find-upgrades)
         :key (lambda (u) (epl-package-name (epl-upgrade-installed u))))
```

Or, without `cl-lib`:

```cl
(let ((upgrades (epl-find-upgrades)) found)
  (while (and upgrades (not found))
    (setq found (eq (epl-package-name (epl-upgrade-installed (pop upgrades)))
                    (epl-package-name package))))
  found)
```

I don't think that this is something we need an extra function for.