feat: reworked poly check command

[DavidVujic]

Description

The poly check command is now analyzing brick imports - local and third-party - and reports if there is something missing in the projects. It no longer runs the poetry check-project command behind the scenes.

Motivation and Context

The reasoning behind the rework can be found in #58

How Has This Been Tested?

Manual testing in REPL, local install and run on example project(s).
CircleCI ✅

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have read the code of conduct.
• I have read the contributing guide.
• I have updated the documentation accordingly (if applicable).