ProjectFileDependencies returns the dependencies listed in a clojure project file.

ShellOutput creates a Shell which shells out to the supplied binary to generate values to return.

ValidBinary attempts to execute a list of `lein` commands and returns the most preferred.

Dependency models a leiningen dependency.

Shell implements Output and uses Cmd to generate the DependencyGraph.

Output is intended to supply a dependency graph from lein output.

level is 1-indexed.