All packages are in ~/.emacs.d/elpa
Package dependencies are listed in files ending with "-pkg.el" in each package directory.
To find which packages require some package, find "-pkg.el"'s that contain that package's name.
Example: find all packages that require "popup" package.
M-x rgrep prompts:
Each dependency is in a list, so the name starts with a "(", then goes package name, one or more spaces, and a double quote(") of a version string.
So, use basic regular expression to find a string in this format:
Search for: (popup \+"
Search for "(popup \+"" in files matching wildcard: *pkg.el
Base directory: /home/somename/.emacs.d/elpa/
A *grep* buffer with links to all packages that depend on popup.
In such situations just
M-x rgrep everything.
I grepped all my Emacs installation for a "contrib" word. And there' a NEWS org file that logs all changes in new versions.
In version 7.9.2, about that "//orgmode.org/elpa/":
You can now add the Org ELPA repository like this:
(add-to-list 'package-archives '("org" . "https://orgmode.org/elpa/")
It contains both the =org-.tar= package (the core Org distribution,
also available through https://elpa.gnu.org) and the =org-plus.tar=
package (the extended Org distribution, with non-GNU packages from the
M-x package-deleteto delete the package, and it will tell you what package depends on it.