When the "library" part of a "plugin" configuration stanza does not
contain at least one path separator, treat it as a filename and assume
it is a name of a shared object present in the named plugin installation
directory. Absolute and relative paths can still be used and will be
used verbatim. Get the full path to a plugin before attempting to
check/register it so that all relevant log messages include the same
plugin path (apart from the one logged when the full path cannot be
determined).
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