Cleaner call of DBT CLI as Python Module

[vicmattos]

Resolves #

Calling dbt.cli module without specifying main module.

Problem

Call DBT cli as module as dbt.cli.main

Solution

Create a __main__.py at the root of core/dbt/cli calling cli() from main.py.

Checklist

• [x] I have read the contributing guide and understand what's expected of me.
• [x] I have run this code in development, and it appears to resolve the stated issue.
• [x] This PR includes tests, or tests are not required or relevant for this PR.
• [x] This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• [ ] This PR includes type annotations for new and modified functions.

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