We last tweaked this logic in rust-lang#13066.
However, we noticed this was inconsistent with `cargo add` in
automatically selecting version requirements.

It looks like this is a revert of rust-lang#13066, taking us back to the behavior
in rust-lang#12950.
In rust-lang#12950 there was a concern about the proliferation of no-MSRV and
whether we should de-prioritize those to make the chance of success more
likely.

There are no right answes here, only which wrong answer is ok enough.
- Do we treat lack of rust version as `rust-version = "*"` as some
  people expect or do we try to be smart?
- If a user adds or removes `rust-version`, how should that affect the
  priority?

One piece of new information is that the RFC for this has us trying to
fill the no-MSRV gap with
`rust-version = some-value-representing-the-current-toolchain>`.

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