[workspace-hack] record exact versions

[sunshowers]

Exact versions work better with dependabot.

With the configuration of dependabot we're running, consider what happens if a dependency (e.g. anyhow) that's in the workspace-hack (anyhow = { version = "1" }) gets updated. In that case, dependabot will add a patch version number (anyhow = { version = "1.0.75" }) to the workspace-hack, and then hakari will fail because it'll want to update it back to anyhow = { version = "1" }.

This change makes that not an issue -- hakari will expect, and want, the version number to be written out as anyhow = { version = "1.0.75" }.

Note that Dependabot isn't long for this world -- with #4166 we'll switch to Renovate, which (among other advantages) obviates this issue. However, we'll probably still want exact versions for uniformity with the rest of the workspace.

[davepacheco]

@sunshowers would you mind summarizing any notes about why this matters? I'm thinking from the context of someone in the future wondering why we made this change, what would break if we wanted to undo it for some other reason, etc. I'm just afraid it won't be the last time we're debugging a dependabot failure.

[sunshowers]

@davepacheco -- done, added to summary. Was in a bit of a rush this morning.

[davepacheco]

Thanks for that!