Improve --add-package duplicate detection

[emmyoop]

Problem

After fixing the core bug in #9104, the check_for_duplicate_packages method still had limitations in its duplicate detection logic:

1. Naming inconsistencies: Hub packages often use underscores (e.g., dbt-labs/dbt_utils) while git repositories use hyphens (e.g., dbt-utils.git), causing the method to miss duplicates across different package sources.
2. False positive matches: The substring matching was too loose, causing unintended matches. For example, adding dbt-core would incorrectly match and remove dbt-core-utils.

These issues made it difficult to reliably detect and remove duplicate packages when using dbt deps --add-package, potentially leading to packages being listed multiple times in packages.yml.

Solution

Flexible Variant Matching

• Generate multiple name variants to handle underscore/hyphen inconsistencies (e.g., dbt_utils ↔ dbt-utils)
• Check both full package names (dbt-labs/dbt_utils) and short names (dbt_utils)
• Enables cross-source duplicate detection: hub packages correctly identify and replace git packages with the same underlying package name

Word Boundary Checking

• Implemented custom word boundary validation using / and . as delimiters
• Prevents false positives where package names are substrings of other packages
• Example: dbt-core will not match dbt-core-utils since - is not a word boundary
• Example: dbt-utils will correctly match dbt-utils.git since . is a word boundary

Enhanced Docstring

• Clarified that this method only runs during --add-package
• Documented the cross-source matching behavior
• Explained the underscore/hyphen variant handling

Note this branched off #12233 and is dependent on those changes. But I separated it out since it's a different fix

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 or 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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.94%. Comparing base (c559848) to head (bb53e7b).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12239   +/-   ##
=======================================
  Coverage   91.94%   91.94%           
=======================================
  Files         203      203           
  Lines       24965    24982   +17     
=======================================
+ Hits        22953    22969   +16     
- Misses       2012     2013    +1     

Flag	Coverage Δ
integration	88.83% <100.00%> (+<0.01%)	⬆️
unit	65.21% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Unit Tests	65.21% <100.00%> (+0.02%)	⬆️
Integration Tests	88.83% <100.00%> (+<0.01%)	⬆️

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