Make module binding look in Main

[tuckermcclure]

Summary

Fix base_module round-tripping for values whose type is defined in an external root module, such as Dates.Date.

Previously, write_to_yaml(...; base_module = C) could emit a type tag like Dates.Date, but load_from_yaml(...; base_module = C) only tried to resolve Dates inside C, causing load to fail unless C.Dates existed.

This updates name resolution so that:

• Names are still resolved relative to base_module first.
• Explicit roots Main, Base, and Core continue to work.
• If the first name is not defined in base_module, the loader can resolve a matching module binding from Main, covering tags like Dates.Date.
• Resolution now uses isdefined before getfield instead of relying on try/catch.

I also updated the README to document the base_module/Main import contract and added a regression test for Dates.Date.

Testing

Added a unit test that failed before making the changes and that works now.