Add strict isOnlyDevDependency() to check for dev dependencies

The current `DependencyEdge::isDevDependency()` is a bit ambiguous with regards to mixed dev- and non-dev-dependencies: Source packages that list the dependency in both "require-dev" and "require" will only create a single edge that is considered to be a dev dependency.

This PR suggests to add a new `DependencyEdge::isOnlyDevDependency()` method to provide a stricter check for when a package is *only* listed in "require-dev" instead.