Rename forward-models to forward_models

[berland]

Better to stick to underscore for consistency with the enforced underscore in the python package hierarchy. Underscore is also consistent with semeio.

Issue
Resolves eye itch

Approach
🧹

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.68%. Comparing base (2a91756) to head (f68b8b0).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8908      +/-   ##
==========================================
+ Coverage   91.61%   91.68%   +0.06%     
==========================================
  Files         343      343              
  Lines       21168    21168              
==========================================
+ Hits        19393    19407      +14     
+ Misses       1775     1761      -14     

Flag	Coverage Δ
cli-tests	39.84% <ø> (+0.01%)	⬆️
gui-tests	73.80% <ø> (-0.03%)	⬇️
performance-tests	50.43% <ø> (+0.02%)	⬆️
unit-tests	80.32% <ø> (+0.05%)	⬆️

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