Support direct Location assignment to variant paths

[lubomir]

When users assign a Location object to variant.paths.repository[arch], the system now properly handles it instead of wrapping it in another Location during serialization.

Changes:

• Modified VariantPaths._serialize_v2() to detect when a value is already a Location object (from direct assignment)
• Store the Location in _locations for v2.0 round-trip fidelity
• Maintain backward compatibility with string assignments

Fixes: #227

[lubomir]

This is marked as a draft, because I'm not sure this is the correct approach. It allows assigning Location directly into the path, but that leads to a weird situation where the variant paths return type can be a string or a Location depending on how the metadata was loaded or created. That is likely going to be confusing.

[guillermodotn]

Witing to _locations during serialization makes serialize() a mutating operation, which can backfire us back.

And if the field dict and _locations both hold a Location but get out of sync, the one in the field dict always wins because isinstance is checked first.

Checking _locations first and falling back to isinstance for direct assignment fixes both.

[lubomir]

Let's close this in favor of #231