precise versions may be overwritten with `--recursive=true` (default)

With #790 being fixed with #795, there are "last writer win" issues where one `cargo update --aggressive` might do the precise version selection from a previous `cargo update`.

I assume we can fix this when integrating `cargo upgrade` into cargo by adjusting the internal API for `cargo update` to meet our need (multiple precise being set and some non-precise being set)

Start by reviewing how cargo update handles precise and non-precise version selections when --recursive=true, then compare that flow with cargo upgrade integration. Trace the internal cargo update API discussed in the issue and verify that multiple precise selections are preserved without later updates overwriting them.

Written by the indexing model from the issue text.