Create `mavenEnv` function

[fzakaria]

Given that a dependency file is now created with all necessary 3P jars and hashes, in the most basic case enough information is provided so that code may likely compile with javac.

Unfortunately, many Java applications rely on maven lifecycle phases to generate code etc.. so keeping maven in the build process is essential.

mvn2nix should create a mavenEnv that takes the dependencies.nix file and creates a symlinkFarm of every dependency into a new Maven style layout in the /nix/store