gnu: emacs-beancount: Move to (gnu packages finance).

This is a fixup for commit abcac7a5, as
inheritance across modules is not safe.  Fixes <https://bugs.gnu.org/40488>.
Reported by Christopher Baines <mail@cbaines.net>.

* gnu/packages/emacs-xyz.scm (emacs-beancount): Move from here ...
* gnu/packages/finance.scm: ... to here.