bug#33870: 27.0.50; xref-goto-xref not configurable

[Juri Linkov]

> Any simplification to the implementation that keeps the
> "keep original window intent" behavior across xref
> intermediate buffers is very welcome.
>
> Juri, do you understand what that particular sub-feature
> is about?

I don't understand what does this "keep original window intent" mean.
Please explain.  I want to understand how it is different from other
modes that display a buffer in another window, such as e.g.
after `C-x C-b' (list-buffers) typing `C-o' displays the buffer
in another window, or e.g. typing `C-o' in the Dired buffer
opens it in another window, and many other cases.

> However here's a tangent that might affect the decision.
> Is there any impediment to making xref.el a core ELPA
> package? I can see some advantages... The reason I bring
> this up here is that using very new elisp in a file can reduce
> the usefulness of that goal, which in this case would be
> to bring new xref features to users of Emacs 26.1/26.2.
> Perhaps it is already using post-26 features in which case
> the ship has sailed.  In that case, disregard this tangent.

Display actions have been in Emacs for a long time customizable
by `display-buffer-alist', so if you need some specific logic,
it's easy to implement the corresponding display action
that can be overridden by the users.