organize: maintain mapping to original filenames and checksums

a request by labs via @bendichter to know which file was converted to the dandiset. perhaps organize can upload this with the file/item metadata and dandi cli can look this up from the server.

however in general perhaps we can add a simple provenance file:

```
wasDerivedFrom .
.
```
this may be useful to create a manifest file later or for checking against amazon store on download.

at present organize doesn't change checksums. so that notion is simply a flag for now.