MPSVirtualFileSystem now is NewVirtualFileSystem and evokes NewVirtualFiles.
Seems, that idea's SearchScope simply ignores a VirtualFile if it's not NewVirtualFile. see DirectoryIndexImpl.getInfoForFile()
Fixed interaction of idea's SearchScope and our psiNodes/VirtualFiles
Fix FileDataSource -> FileSystemBasedDataSource.
Now getParent() works correctly for ModelVirtualFile whose model is has per-root data source.
Better usages view in idea plugin