Add setting to fail 'tach check' if circular dependencies exist

[emdoyle]

Some projects would like to ensure that their project's dependencies remain unidirectional, or in other words, that there are no cycles in their dependency graph.

A simple way to begin supporting this is to add a setting like forbid_circular_dependencies which can be set in tach.yml (ProjectConfig within the Tach codebase). When this setting is true, tach check should fail with a specific error message when a circular dependency is detected.

This should likely be done as a "pre-flight check" which would validate the modules list, and could be delegated to the ModuleTree internally. It does not need to apply any new logic to actual detected imports.

[tiluckdave]

Hey @emdoyle can I give this one a try next?

[emdoyle]

Absolutely!