feat: Support mixed MSRV in --version-range

[epage]

This expands on the approach taken in #212 bucketing packages into rust-versions to run. If we skipped the MSRV (due to --version-step), we automatically inject it. If a package's MSRV isn't within the range, we skip it.

Benefits

• Relatively simple to implement and to explain
• We keep the number of runs to a minimum by walking in lock-step the --version-step, independent of what each package' MSRV

I did have to specialize --rust-version vs --version-range to avoid --rust-version range users walking more than they needed.

To keep the progress total accurate, I shifted the calculating of the total from determine_package_list to after we have bucketed everything. To make this feasible, I saved off the how many iterations a package will have without the version range being taken into account.

As a byproduct, this fixes a bug in #212 where it didn't take the rust-version into account when determining the total.

Fixes #199

[epage]

@taiki-e CI is passing

[taiki-e]

Great! Could you add a test case for --version-step?

[epage]

I also should add a warning when a selected package is skipped due to msrv being higher than the end of the range in case it isn't intentional

[epage]

CI is passing again with the applied changes

[taiki-e]

Published in 0.6.6. Thanks again @epage!