chore(custom-checks): add check to prevent circular imports

[glevco]

Motivation

Sometimes, when something is imported from the hathor module in an entrypoint file (from CLI commands), a circular import could happen, causing the full node node startup to fail. This has happened a couple times in the past (example).

This problem would not be caught by our linters and neither tests, so it would only appear after the merge to master, when the docker image is generated. This PR aims to prevent this by adding a custom lint check that prohibits importing from hathor in the module-level of CLI entrypoints. For specific cases, a # no-custom-check comment can be added to opt-out of this check. Here's an example of a failing output.

This will also be useful in #889, as it prevents indirect calls that could automatically install the default Twisted Reactor before our code had a chance to customize it.

Acceptance Criteria

• Add new custom check to prevent importing from the hathor module in the module-level of CLI related files.
• Update code to conform with the new check.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7dded42) 85.33% compared to head (de67a6b) 85.37%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #887      +/-   ##
==========================================
+ Coverage   85.33%   85.37%   +0.04%     
==========================================
  Files         282      282              
  Lines       22280    22280              
  Branches     3366     3366              
==========================================
+ Hits        19012    19021       +9     
+ Misses       2596     2588       -8     
+ Partials      672      671       -1     

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