I've been using this for years, but never bothered to make it public until now.

el-rcfiles is a very small and simple library which provides Unix-like RC files for Emacs Lisp libraries. It's compatible with GNU Emacs and XEmacs, available in ELPA form, as a tarball and from GitHub. More details (including download) available here, but here is also the library's commentary section, for quick reference.

;;; Commentary:

;; The purpose of el-rcfiles is to provide the equivalent of traditional
;; Unix rc files (i.e. configuration files) for Emacs Lisp
;; libraries. The advantages of using configuration files are the
;; following:
;;   - your initialization file is less bloated,
;;   - since configuration files are lazily loaded, your Emacs session
;;     is (or begins) lighter. That is unless you already use lots of
;;     EVAL-AFTER-LOAD forms...

;; Usage:

;; 1. Load the library, go to the rcfiles Custom group and tweak (or not).
;; 2. Put a call to (rcfiles-register-rc-files) in your initialization
;;    file. This function can also be called interactively anytime you
;;    add, remove or modify a configuration file.
;; 3. Put your configuration code for a library `foo' in a file called
;;    `<rcfiles-directory>/foo<rcfiles-pseudo-extension>.el'.