Automatically use `webExt.sourceDir` from package.json

[fregante]

Instead of changing the CWD or using --source, every WebExtensions tool could follow Mozilla’s web-ext setup if present: https://extensionworkshop.com/documentation/develop/getting-started-with-web-ext/#automatic-discovery-of-configuration-files

The closest package.json can be found via:
https://github.com/sindresorhus/pkg-up