Ionic Package not building plugin with Git spec

I’m using Ionic Package to build for iOS. One of the Cordova plugins my app uses isn’t on NPM, so it was installed from Github. I have this in my config.xml:

<plugin name="com.outsystems.audiorecorder" spec="" />

And I have a folder com.outsystems.audiorecorder in the plugins folder, which contains the source for both Android and iOS, just the same as any other plugin.

The problem is that the output spat out by Ionic Package doesn’t include this plugin.

How can I get Ionic Package to build plugins from Github, or indeed custom plugins stored locally (e.g. not on NPM or Github)?