use-package: local package file

[Trevor Arjeski <tmarjeski@gmail.com>]

Hi all,

I'm obsessed with containing everything inside of (use-package ...)
forms, and I've been trying to figure out an elegant way to "lazy" load
a local package.

For example, I keep my personal elisp package files at ~/.emacs.d/lisp/,
which is a directory that is added to `load-path'. I would like to write
a use-package form for a package located at `~/.emacs.d/lisp/foo.el':

(use-package foo
             ...)

and have package.el automatically install all of foo.el's dependencies,
listed under `Package-Requires' in the header.

So far, I figure out that overriding `use-package-ensure-function' with
a custom function that checks if a library can be located and is not
installed, as follows:

(defun my/use-package-ensure (name args _state &optional _no-refresh)
    "Checks for local package before checking remote archives."
    (if-let* ((path (locate-library (symbol-name name)))
              (_ (not (package-installed-p name))))
        (package-install-file path)
      (use-package-ensure-elpa name args _state _no-refresh)))

This somewhat works, but for some reason I receive the error on Emacs startup:

     error: Package ‘foo-dependency’ (version 1.0.0) is unavailable

...when, in fact, the version is available from nongnu and melpa.

`package-install-file foo' works perfectly fine when called
interactively after Emacs starts, so I suspect something isn't loaded in
time when `use-package-ensure-function' is called.

Any advice would be appreciated. Thanks.