[prefs] Better define behaviour of #nodeExists for project nodes

This is a migration of [Bug 90500](https://bugs.eclipse.org/bugs/show_bug.cgi?id=90500):

> DJ Houghton CLA Friend 2005-04-06 13:51:36 EDT
>
> We need to better define the behaviour of #nodeExists for the nodes in the
> project scope.
>
> Currently we over-ride super-class impl such that the node does exist if the
> super-class says that it does or if you are at /project and you are asking about
> a single segment name and that name is the name of a project that exists in the
> workspace.
>
> The impl doesn't work for long paths though since super accesses the cache of
> children. (e.g. nodeExists("myProject/qualifier/nodeName")
>
> Investigate the interaction between #nodeExists and #getChild, #getChildren, and
> #childnames.

which is still referenced at

https://github.com/eclipse-platform/eclipse.platform.resources/blob/88f58177bf0ad15355d3d1136c5d8b4aac7be751/bundles/org.eclipse.core.resources/src/org/eclipse/core/internal/resources/CharsetManager.java#L363-L384

we either should fix this or remove the reference from the code