Improve diagnostics for local dependency and environment issues

[basnijholt]

Summary

• make the Conda environment mismatch warning actionable and keep installs targeted at the active Conda prefix
• detect duplicate local package sources before pip/uv fail with conflicting direct-reference errors
• improve local dependency diagnostics for missing paths, empty directories, and empty Git submodules

Testing

• uv run --extra test python -m pytest tests/test_cli.py tests/test_setuptools_integration.py tests/test_utils.py tests/test_parse_yaml_local_dependencies.py tests/test_pypi_alternatives_errors.py tests/test_cli_install_conda_lock.py --no-cov -q
• pre-commit run --files unidep/_cli.py unidep/_dependencies_parsing.py unidep/_setuptools_integration.py unidep/utils.py tests/test_cli.py tests/test_setuptools_integration.py tests/test_utils.py tests/test_parse_yaml_local_dependencies.py tests/test_pypi_alternatives_errors.py

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📚 Documentation preview 📚: https://unidep--278.org.readthedocs.build/en/278/

[codecov]

Codecov Report

❌ Patch coverage is 91.13208% with 47 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.69%. Comparing base (fa42bc9) to head (30bf425).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
unidep/_cli.py	85.21%	21 Missing ⚠️
unidep/utils.py	94.84%	12 Missing ⚠️
unidep/_dependencies_parsing.py	89.42%	11 Missing ⚠️
unidep/_setuptools_integration.py	93.87%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #278      +/-   ##
==========================================
- Coverage   98.07%   96.69%   -1.38%     
==========================================
  Files          11       11              
  Lines        2285     2757     +472     
==========================================
+ Hits         2241     2666     +425     
- Misses         44       91      +47     

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