extract_source_files plugin doesn't record the non-existence of a file

A full run of the ExtractSourcePackageFiles task is really long, even after you have already run it once. The (likely) reason is that it does re-extract many sources packages where one of the file to extract does not exist (i.e. the watch file since this is the only optional file), just to see that in fact it doesn't exist.

We should really record the fact that we have tried to extract a given file but that it doesn't exist. This can be stored in the task data or in the ExtractedSourceFile model, possibly with an existing attribute that would be set to False in those cases (and then the extracted_file attribute would be NULL). The latter is probably best but will require changes in other places than the task code.