Fix loading plugins using just their filenames

BIND 9 plugins are installed using Automake's pkglib_LTLIBRARIES stanza, which causes the relevant shared objects to be placed in the $(libdir)/@PACKAGE@/ directory, where @PACKAGE@ is expanded to the lowercase form of the first argument passed to AC_INIT(), i.e. "bind". Meanwhile, NAMED_PLUGINDIR - the preprocessor macro that the ns_plugin_expandpath() function uses for determining the absolute path to a plugin for which only a filename has been provided rather than a path - is set to $(libdir)/named. This discrepancy breaks loading plugins using just their filenames. Fix the issue (and also prevent it from reoccurring) by setting NAMED_PLUGINDIR to $(pkglibdir).