Unified project interface

[Dmitry Gutov]

Hi all,

A while ago, there was floated a "unified project root" proposal. I'd 
like to resume that discussion.

Unfortunately, the code proposed back them doesn't serve the immediate 
need I have in mind: basically, I need that function to return a *list* 
of directories, because both etags and elisp xref backends operate on 
multiple directories, via tags-table-list and load-path respectively.

If we have it, we'll be able to untie `xref-find-regexp' from xref 
backends, because really there's nothing language-specific to this command.

Now, before adding this feature to the core, are there other related 
general questions we want to be able to answer? Depending on that, a 
"project" might be better implemented as a cl-struct, providing 
specialized implementations for those several generic methods.