On Windows systems the plugin binaries use a .exe suffix, which we were
misparsing as part of either the "v" or "x" parts of the filename.
This fixes#15578.
For now this supports both our old and new directory layouts, so we can
preserve compatibility with existing configurations until a future major
release where support for the old paths will be removed.
Currently this allows both styles in all given directories, which means we
support more variants than we actually intend to but this is accepted to
keep the interface simple such that we can easily remove the legacy
exception later. The documentation will reflect only the subset of
path layouts that we actually intend to support.
Martin Atkins