new_post: log .meta path only when it exists (fix #3731)

[Ayudh-M]

Pull Request Checklist

• I’ve read the guidelines for contributing.
• I updated AUTHORS.txt and CHANGES.txt.
• I ran flake8 nikola tests and pydocstyle … — no new warnings.
• I built the demo site (nikola init -qd demosite && nikola build) and tested nikola new_post in the scenarios below.

Description — fix #3731

nikola new_post --import printed a misleading line about a “metadata” file even when the post was created in one‑file mode (or when the .meta file did not actually exist).
This tiny patch:

• performs the duplicate‑title check more explicitly (exists_txt, exists_meta)
• logs the .meta path only if it really exists
• keeps the previous behaviour for two‑file posts
• removes dead, duplicated code and satisfies flake8/pydocstyle

No functional changes beyond quieter, more accurate logging.

Testing

command	expected / observed
nikola new_post --title "X" --import foo.rst (new title)	post created, no error
repeat the same command	The title already exists! + text‑path only
repeat with --twofile	same error + text‑ and meta‑paths logged

Everything behaves as described, both with ONE_FILE_POSTS = True and False.

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Kind request: this is for a university assignment due 19 April.
The patch is small (±20 lines) and isolated; even a quick “LGTM / needs X” reply would be immensely helpful for my report. Thank you very much for your time!