Extract graph-based decl dependency code into its own module

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.63556% with 217 lines in your changes are missing coverage. Please review.

Project coverage is 82.11%. Comparing base (076cf2e) to head (ea83cb1).

Files	Patch %	Lines
src/Escalier.TypeChecker/Graph.fs	72.63%	217 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #221 +/- ## ========================================== + Coverage 82.09% 82.11% +0.02% ========================================== Files 21 22 +1 Lines 10311 10324 +13 Branches 1516 1516 ========================================== + Hits 8465 8478 +13 Misses 1846 1846 ```

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