el-get-set-info-path: make safe if package removed

Use an eval-after-load form that won't throw an error, regardless of
removed packages.

Fixes #1562. I didn't use the hack suggested there, since that would depend on the precise form `eval-after-load` adds to `after-load-alist` which could easily change between emacs versions.

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Note that the info path still isn't removed on package uninstall; I looked at how `infodir-rel` is computed:
```el
      (let* ((infodir-abs-conf (concat pdir infodir))
             (infodir-abs (file-name-as-directory
                           (if (file-directory-p infodir-abs-conf)
                               infodir-abs-conf
                             (file-name-directory infodir-abs-conf))))
             (infodir-rel (if (file-directory-p infodir-abs-conf)
                              infodir
                            (file-name-directory infodir)))
             (info-dir    (concat infodir-abs "dir"))
             (infofile (if (and (file-exists-p infodir-abs-conf)
                                (not (file-directory-p infodir-abs-conf)))
                           infodir-abs-conf
                         (concat infodir-abs pname))))
```
too complicated, depends on state of the filesystem: a good chance we'd come up with a different answer during uninstall.