* sly.el (sly--when-let): Renamed from `when-let'.

* contrib/sly-mrepl.el (sly-mrepl--insert-prompt)
(sly-mrepl--open-dedicated-stream): Use `sly--when-let'

* contrib/sly-package-fu.el (sly-goto-package-source-definition): Use
`sly--when-let'