Utilize *.json `"url"` field as unique image identifier for Google Takeout imports

[dschwen]

The "url" field in the .json sidecar file is a direct link to the image on Google Photos, and it thus constitutes a unique identifier for the photo (regardless of filename duplication).

A map could be constructed from that URL to the image file next to the sidecar. Sidecars appear multiple times in the export (in each album folder the image appears in), but the actual images should appear only once (mostly in "Photos from ****", but also in family and friends type folders). The map would thus point from the unique URL to the actual location of the image file in the dump.

[simulot]

Got it. Still working on the refactoring. I'll use that