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'.