Currently, a buffer for an archive file can be opened by selecting the
entry from archive-mode, but there is no way to access it from Lisp
without manually selecting the entry.

An API like archive-visit-inner-file, introduced in this patch, would be
useful to implement the ability to directly access an archive entry.

My use case is to be able to jump directly to classes and functions
defined in PHP scripts stored inside a Phar archive file.

- https://github.com/emacs-php/archive-phar.el
- https://www.php.net/manual/en/book.phar.php

I'm not hung up on the function names or detailed API of this patch.