The plugin that I found that has this feature is called Open File in Solution
Here's a link to download the plugin and documentation:
Using the Notepad++ Plugin Manager, you can download the plugin directly from there (and it'll download the dependencies: SolutionHub and SolutionHub UI)
Before you can use the plugin however, you need to do some setup in SolutionHub UI which is detailed at the link above.
Then you can create a new Shortcut mapping to bind the plugin to a key.
I've been using it for the past few hours, it's really fast. Happy I found this.