<c:list-property name="*" displayName="Installed extensions" readOnly="true" required="false">
<c:map-property name="*" displayName="Name" readOnly="true">
<c:simple-property name="module" displayName="Module name" readOnly="true"/>
Luckily IDEA offers "Find by XPath"
So I could type in my expression and have it search through the whole project.
This looked good, but did not work. Then I remembered the little 'c:' and clicked on the "Edit context..." button, which allowed to set the correct namespace for 'c:':
Now with the namespace correctly set, the search ran for a few seconds and found the occurrences all across my files. /me is a happy user.