Move graph walkers to separate module

[JCZuurmond]

Changes

Move all graph walkers into a separate module graph_walkers.py under the linters module to centralize them, because they are used for linting only and to reuse them instead of recreating them

Linked issues

Progresses #3514
Breaks up #3520

Functionality

• modified existing command: databricks labs ucx migrate-local-code

Tests

• manually tested
• modified and added unit tests
• modified and added integration tests

[github-actions]

✅ 57/57 passed, 2 skipped, 41m55s total

_{Running from acceptance #8152}