Set the src.origin property on the imported file

Currently, the imported file has no information about where it was originated. This can impact log messages and make it more difficult for other extensions to know how to process the file. The imported file should have a src.origin property that's consistent with files discovered by the content aggregator.

The origin object should be the origin on which collector was run. It should also contain a flag named generated to indicate that the file was generated. This flag allows other software to know that the file won't actually be found in the origin (i.e., content root).

The temporary worktree on which collector is run is not useful since it gets cleaned up after the collector extension runs.