Evaluate the approach underlying Graal

[valeriocos]

Graal contains 6 backends that allow to fetch different kind of information from source code (complexity, licenses, dependencies, etc.). Each backend can fetch source code information using different tools. For instance, coqua relies on pylint and flake8 for code quality, colic leverages on nomos and scancode to get license insights. Every time a backend is enhanced to support a new tool, the corresponding code looses cohesion and the maintenance/evolution effort increase.

The goal of this ticket is to evaluate a different approach for graal backends, where each backend relies on a single tool. In this case scenario, there will be a backend for flake8, pylint, nomos and scancode.