External plugin loader support for Windows is introduced
in this dev cycle. Since helper binary was not required (useless)
before this version, people may not ship the binary
with new GStreamer version, then they will observe warning message.
Instead of displaying the warning at plugin loading time,
checks helper bin earlier and disable external plugin loader
if helper binary is not installed.
Fixes: https://gitlab.freedesktop.org/gstreamer/cerbero/-/issues/448
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6083>
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