Convert from WritableManifest -> Manifest to represent deferred manifest internally

[MichelleArk]

resolves https://github.com/dbt-labs/dbt-core/issues/9567

Problem

Solution

Checklist

• [ ] I have read the contributing guide and understand what's expected of me
• [ ] I have run this code in development and it appears to resolve the stated issue
• [ ] This PR includes tests, or tests are not required/relevant for this PR
• [ ] 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

[github-actions[bot]]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.52941% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 88.01%. Comparing base (7329143) to head (6b70358).

Files	Patch %	Lines
core/dbt/contracts/graph/manifest.py	93.33%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9544      +/-   ##
==========================================
- Coverage   88.01%   88.01%   -0.01%     
==========================================
  Files         178      178              
  Lines       22375    22413      +38     
==========================================
+ Hits        19694    19727      +33     
- Misses       2681     2686       +5     

Flag	Coverage Δ
integration	85.47% <98.52%> (+<0.01%)	:arrow_up:
unit	62.23% <95.58%> (+0.07%)	:arrow_up:

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