make `nextVersion` work with multiple versions

**Is your feature request related to a problem? Please describe.**

When you use independent versioning, the logic in `shouldPrepare` needs to depend on multiple versions, not just one

**Describe the solution you'd like**

both shouldPrepare and version should deal with multiple version numbers, possibly an array of package name and version.

**Describe alternatives you've considered**

Ignore shouldPrepare in those cases, and always prepare, even if there's nothing to release

**Additional context**

we are using "monorepo with independent versioning" that skips most of the features of `shouldVersion` in favour of `version`, but shouldPrepare still gets called

Start by tracing the shouldPrepare and version entry points and how independent versioning currently supplies a single version. Define the multiple-package/version representation and verify that preparation decisions use all versions, including the no-release case, with focused tests for independent versioning.

Written by the indexing model from the issue text.