pathMapping does not work when launch configuration in user settings.json and project in devcontainer

This finds `pathMapping` when defined in `launch.json`. However it does not work, when I place it under the `launch` key in my user `settings.json`:

```
{
// Use IntelliSense to learn about possible attributes.
// Hover to view descriptions of existing attributes.
// For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
"version": "0.2.0",
"configurations": [
{
"name": "Firefox Debugger",
"type": "firefox",
"request": "launch",
"reAttach": true,
"profile": "dev",
"keepProfileChanges": true,
"url": "http://localhost:8080/",
"skipFiles": [
"${workspaceFolder}/app/node_modules/**"
],
"pathMappings": [
{
"url": "webpack:///src",
"path": "${workspaceFolder}/app/src"
}
]
}
]
}
```

The extension offers to do a `pathMapping` like the following. This actually works, but it is not generic.

```
{
"url": "webpack:///src/components/Component.vue",
"path": "vscode-remote://dev-container%2B2f686f6d652f6272616e64732f7265706f732f626574612d617070/project/app/src/components/Component.vue"
},
```

### Expected

The launch configuration should also work in `settings.json`.