libgimpbase: remove case of plug-ins binaries directly under plug-ins/.

In GIMP 3, plug-ins are mandatorily under a subdirectory. Though on GIMP
2.10.x, both cases are still possible. So only the previous commit
should be backported to gimp-2-10 branch.

See discussion in !392.
Thanks also to Christopher Nielsen for helping.
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build-image

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build-image-win32

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deps-debian

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  Gimp
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gimp-autotools-debian

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gimp-clang-debian

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gimp-distcheck-debian

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gimp-meson-debian

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cppcheck

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  Distribution
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sources

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