I have not found any existing solution to do that and come up with my own:
;; Inspired by https://www.emacswiki.org/emacs/BookmarkPlus#BulkDownloadCompileLoad
(defun fetch-and-load-elisp (pkg-name pkg-files base-url base-dir)
(let ((pkg-dir (concat (file-name-as-directory base-dir) (symbol-name pkg-name))))
(add-to-list 'load-path ...
Reading the source code (of multiple-cursors) helped me with this one. :)
This key is defined in file multiple-cursors-core.el and the end of this file provides the feature multiple-cursors-core, so you have to use-package this feature.
("<return>" . nil)))
The answer is present on use-packages's GitHub page under section "Notes about lazy loading". To quote,
In almost all cases you don't need to manually specify :defer t. This is implied whenever :bind or :mode or :interpreter is used. Typically, you only need to specify :defer if you know for a fact that some other package will do something to cause your ...