[fuchsia] Update product-bundles script to stay consistent with ffx.

`ffx product-bundle get` will add repositories after downloading,
and will not download a product if the repository is still present.
However, `ffx product-bundle remove` does not remove repositories,
preventing a `remove`->`get` on the same product-bundle from
succeeding.

Adding logic to check if a repository is stale (by checking if the
path is present) helps with this.

With a way to confirm that a PB is or isn't downloaded now, we can
verify the success of this by simply getting the list of available
PBs via repository, and comparing this against the desired PBs.
This allows for simplified downloaded and staleness checks (all while
guaranteeing the PB is actually downloaded when we say it is).

This CL also changes how versioning and caching is done. Versioning
(IE, signature) was done via a .product_bundle file. This file is no
longer used by FFX. This also introduced another way for stale
references that don't get updated on remove. Instead, versions
will be determined based on the `product-bundle list` of URLs.

Also adds unit tests to the new methods.

Change-Id: I2b887a252b081ba7f7ed20a53f54b2a6c8062207
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4027068
Commit-Queue: Rohan Pavone <rohpavone@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1075226}