How can I enable the built in package 'IMGUI' in my package manager?

I believe this happened to me when I updated Unity from 2017 to 2018.2.10f1 and noticed the new Packages/manifest.json had a bunch of entries I didn't want, like Ads, Analytics, etc. (what the heck Unity...) So I just removed all entries in there, and then my package manager was empty like yours when I found out I needed to enable the Audio package to compile my game.

The fix: I just deleted the Packages folder, and then Unity will re-generate Packages/manifest.json - you may have to re-open Unity. The first package in the list is "Package Manager UI, which I'm guessing is needed for the package manager window. (There should be an error message for this...) From here I just manually removed all packages I don't want (most of them). Since removing/disabling them individually in Package Manager takes forever to load, I just edited Packages/manifest.json directly which was much faster. In the end I only needed Audio and IMGUI (for now).