repo: drop_data_index: delete all first level nodes

[efiop]

This resulted in outdated fetch index cache and misc errors that are cleared by removing site_cache_dir manually. So because of the latter fact, also bumping site_cache_dir salt here to force refresh.

Likely related #10030

More tests coming separately.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
dvc/repo/__init__.py	94.11% <66.66%> (-0.50%)	⬇️

... and 38 files with indirect coverage changes

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[dberenbaum]

@efiop Could you explain more so I understand the changes? Was it not deleting any of the nodes before? Why do only the first-level nodes get dropped?

[efiop]

@dberenbaum It was only deleting tree prefix. Logical thing to do here instead is delete_node(()) to delete root node, but sqltrie has an assumption that root always exists (and probably rightfully so), so we have to just delete all first level entries instead (deleting a node means that you are breaking the traversal and descendants become orphaned, so we don't need to waste time deleting them here right now). There are a few small things like that about root that bother me, but didn't want to address them here right now.