fix: find dotenv from direct thread targets

[ryanchou1994]

Fixes #531.

When load_dotenv is used directly as a threading.Thread target, the first caller frame belongs to the standard library's threading module. find_dotenv() therefore starts its search next to threading.py and misses the application's current-directory .env.

This change recognizes the actual standard-library module globals by identity and uses the current directory for that direct-target case. Calls through a user wrapper still search relative to the wrapper file, and a user module merely named threading is not mistaken for the standard library.

Tests cover the direct thread target, wrapper behavior, and the same-name module regression.

Validation:

• Linux Python 3.13: 257 passed, 1 skipped
• Ruff check and format check
• mypy for Python 3.10 through 3.14

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