Deleting packages doesn't break the indentation sync

* slynk.lisp (handle-indentation-cache-request): Handle case
where PACKAGE was deleted, would otherwise error and terminate the
indentation sync thread.
Small refactoring and add sly-stickers tests

* sly.el (sly-flash-inhibit): New variable.
(sly-flash-region): Use it.

* contrib/sly-stickers.el (top): Require 'sly-buttons
(sly-stickers--stickers-in, sly-stickers--stickers-between)
(sly-stickers--stickers-exactly-at, sly-stickers--stickers-at)
(sly-stickers--sticker-priority): Move to sly-buttons.el.
(sly-stickers--set-face, sly-stickers--sticker)
(sly-stickers--sticker-substickers)
(sly-stickers--decrease-prio, sly-stickers--increase-prio)
(sly-stickers-clear-region-stickers)
(sly-stickers-delete-sticker-at-point)
(sly-stickers-maybe-add-sticker)
(sly-stickers-compile-region-aware-of-stickers)
(sly-stickers-after-buffer-compilation): Use `sly-button-*' functions.
(sly-stickers--populate-sticker): Also record case when no values were returned.

* lib/sly-buttons.el (sly-button--overlays-in)
(sly-button--overlays-between, sly-button--overlays-exactly-at)
(sly-button--overlays-at, sly-button--overlay-priority): New functions.

* lib/sly-tests.el (defun): No need to check for ert.el.

* test/sly-stickers-test.el (sly-stickers): New file.