- Adds the graphhopper package.nix to nixpkgs -> This is an open source
routing engine written in java and built with maven using the
"double-invocation" pattern - see `doc/languages-frameworks/maven.md`.
- Additionally, comes with a patch to it's pom.xml because it includes
[frontend-maven-plugin](https://github.com/eirslett/frontend-maven-plugin),
which tries to install node/npm during the build phase, which does not
work. Hence, we fetch the single npm package required manually.
