Resolve case of shared file use in multiple Dandisets

[samuelbray32]

• Currently, the dandi export process creates a unique Dandi upload for each raw and analysis file in its dataset. (see #956)
• This would create redundant entries in the Dandi database if files are re-used between projects.

Potential fixes:

• Investigate dandi methods for linking to files between dandisets
• If not, redundant uploads are required. Resolve DandiPath.fetch_file_from_dandi() case where multiple versions of the file are present