Split ocrmypdf.subprocess/__init__.py into three private submodules by
concern (_run, _version, _check) and reduce __init__ to re-exports.
Introduce ocrmypdf._exec._probe.ToolProbe to centralize the version()/
available() pattern each tool module was reimplementing, so the "is this
tool installed and suitable?" question is cleanly distinct from the
pure, picklable functions that do the work.
Also replace the ghostscript module-import log.addFilter() side effect
with an idempotent _ensure_log_filter_installed() called at the top of
each work function, so the DuplicateFilter is present in subprocess
workers without relying on import-time ordering.
Public API of ocrmypdf.subprocess is unchanged.
