Support Gradle files with no top level build.gradle file

[zbynek]

Fixes #3908

Allows fetching and parsing projects with missing top level build.gradle file, as long as they have one or more subprojects defined in settings.gradle.

Manually tested by downgrading one dependency and letting dependabot update it again: https://github.com/zbynek/selenium-webdriver-java/pull/1/files

[jurre]

This looks great, thanks again @zbynek

Small request for future PRs: if you could split up any refactoring into separate commits from the ones that add/change functionality it would make reviewing a little easier 🙇