Improve message on Manifest lock changes check

The `Package.swift` changed warning wording can be misleading because, for a whole class of changes, it does not affect the `Package.resolved` content.

In the lack of smarter ways (without a lot more effort) to detect such cases, we could rephrase the message to something like:

> ⚠️ Package.swift was changed but its corresponding Package.resolved was not. Please ensure Package.resolved is up to date if any of the dependencies in Package.swift changed.

Locate the check that emits the warning about Package.swift changing without a corresponding Package.resolved change, then review its current wording. Update the warning to acknowledge that some Package.swift changes do not affect resolved dependencies, and run the relevant project tests to confirm the message is correct.

Written by the indexing model from the issue text.