cmd/go/internal/workcmd/sync: investigate cases where sync doesn't work correctly with unreleased workspace modules

[matloob]

The aim of this issue is to collect cases where go work sync doesn't work correctly with workspace modules that haven't yet been released.

Two things that mentioned on #50750:

• go work sync tried to find modules to satisfy imports that are in other workspace modules.
• Workspace modules after a sync could have extraneous dependencies. (If there are missing requirements, they can't be "tidy", but we do run the "tidy" logic while allowing errors, and this should work as expected.)