There is no way to do this (in Confluence either - what you've seen for Confluence is a way to find unused macros that may well be provided by addons, but not the full range of all add-ons). The problem is that an add-on can do all sorts of things, and the way they're implemented makes an across-the-board search impossible.
As an example, I often use the Script Runner to provide scripted fields and post functions. They have nothing in common to search for. I can find fields by reading the list of fields, or looking in customfield in the database. To find post functions, you have to read workflows one way or another. I can't even search for "is script runner used" in one question, let alone any other add-on.
I'm afraid you've got three basic options:
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