Do not use copy-of for mapping a container field

[igarashitm]

Describe the Bug

It only works if both source and target are completely same including namespace, which we can't expect for data mapping. We should instead construct parent structures and then put value-of for the terminal fields.

#1780 and #1846 are related. If we don't use copy-of and construct the container structure, we might be able to do a bit more flexible mapping, for example if source side and target side has different children, only map the children that has the same name. And topmost parent name doesn't have to be same on source and target.

Steps to Reproduce the Bug or Issue

1. DnD container field

Screenshots or Videos

No response

Platform

• OS: [e.g. macOS, Windows, Linux]
• Browser: [e.g. Chrome, Safari, Firefox]
• Version: [e.g. 0.30.0]