Redesign structure of org-roam-bibtex package

[myshevchuk]

A continuation of thoughts expressed in #13

1. orb-core.el as a central dependency point. It requires org-roam and bibtex-completion. It is supposed to be required by user interface libraries. It contains defgroup definitions and general utility functions that depend on org-roam, bibtex-completion and other packages not included in Emacs. It itself does not require any user interface libraries but may require utility libraries.
2. orb-utils.el - general purpose utilities that do not depend on extra packages, only built-in Emacs libraries (an exception can be made for general purpose extra libraries such as dash, f, s). If a function requires anything from org-roam, bibtex-comletion or other specialized libraries, it should go into orb-core.el or a specialized utility library.
3. user interface libraries - multiple entry points into the package. They depend on orb-core, orb-utils and perhaps on other user interface libraries. However, if such a library depends on another user interface library, then that code should probably be refactored into orb-core or orb-utils or yet another utility library.