Revert some changes in cross-ref collection

Cross-references has some dependencies in Weaver: cross-ref data is collected in additional sources directory with C files, which are used for coverage. And additional sources for every auxiliary file is used in cache. So, we cannot remove the directory in case of disabled cross-ref. Though, for now just disable .idx files, but potentially additional sources may be also optimized.

Also, fix a path of task description to plugin work directory.