Allow valid targets alongside empty directories

[nightcityblade]

Fixes #22173

create_source_list now defers the existing empty-directory error until all explicit targets have been scanned. This lets mypy check valid targets alongside an empty directory while preserving the fatal error when no sources are found.

A regression test covers both the mixed-target and all-empty cases.

Validation:

• TMPDIR=/tmp .venv/bin/python -m pytest -n0 mypy/test/test_find_sources.py -q (10 passed)
• .venv/bin/python runtests.py lint (passed)
• .venv/bin/python runtests.py self (346 source files checked successfully)
• .venv/bin/python -m mypy .github . against the issue reproduction (success; 1 source file)

AI assistance: Codex helped draft the focused change and test; I reviewed the diff and ran all validation locally.

[nightcityblade]

Fixed the Windows-only test expectation in 7d71289 by accepting both POSIX and drive-prefixed Windows separators. I also folded pre-commit.ci’s import-spacing auto-fix into the authored commit, preserving single-commit history. The focused 10-test suite passed before the formatting-only bot change; the final tree passes Ruff, ruff format, py_compile, and git diff --check. CI is rerunning.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅