* lisp/emacs-lisp/package.el: Fix use of `find-library-name`

* lisp/emacs-lisp/package.el: Fix use of `find-library-name`

That function caused a warning for a good reason.
Don't just declare it and hope it will be available.

(package--list-of-conflicts): Require `find-func` explicitly before
declaring the function.  Also don't ignore all errors but only
the `file-error`s which will be emitted by `find-library-name`
in normal circumstances.

* lisp/emacs-lisp/find-func.el (find-library-name): Signal a `file-error`
Instead of a generic `error`.