Implemented a mechanism to run a dependency graph

This MR implements the calculation of dependence graphs for packages which are hosted in gitlab.

The command is available via bdt gitlab graph --help.

It recursively tries a package resolution via the conda-build API.

The user can choose the format of the output, which types of dependencies to track (among host, build, run and test) and what to put on the graph via a regular expression.

The user needs access to the gitlab API to use the command.

It replaces the old bob_dependency_graph command in bob.extension, that is now removed (see bob.extension!106 (merged)).

Partially addresses #43 (provides the adjancence matrix required for the evaluation of the DAG).