Add strict isOnlyDevDependency() to check for dev dependencies

[clue]

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.

[Potherca]

This would be nice to have... will this be merged any time soon or do we need to use a fork?

[gpupo]

This will be done?