[Discussion] Use etags in ghci instead of hasktags

ghci has a `:etags` (specially for Emacs) feature that generates the `TAGS` table for the loaded modules. The only disadvantage is that `etags` can be generated for working code and then be used, whereas `hasktags` can generate `TAGS` even when the code does not compile. The advantage of using `etags` is that there is no additional dependency as `:etags` comes with ghci by default (even older versions). We could make `:etags` as a fallback for `hasktags` when `hasktags` are not installed.