Demote missing implicit script warning

[juliosuas]

Summary

• demote the missing implicit script_dir message from warning to debug
• keep explicit invalid script_dir metadata as an installation error
• add a regression assertion for plugin-only packages without top-level scripts

Fixes #199.

Testing

• uv run pytest -n 0 testing/test_btest.py -k "tests.load and not dependency"
• uv run pytest -n 0 testing/test_manager.py testing/test_source.py --tb=short
• pre-commit: trailing whitespace, EOF, YAML, large files, Ruff, cmake-format, and cmake-lint passed (mypy 2.1.0 hit an internal error on Python 3.13)

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Restored after an accidental fork deletion. Same commits as #251.