Mappings: Fix `copy_to` behavior on nested documents.

Today, `copy_to` always copies a field to the current document, which is often
wrong in the case of nested documents. For example, if you have a nested field
called `n` which has a sub-field `n.source` whose content should be copied to
`target`, then the latter field should be created in the root document instead
of the nested one, since it doesn't have `n.` as a prefix. On the contrary, if
you configure the destination field to be `n.target`, then it should go to the
nested document.

Close #6701
Maybe add a default constructor for the places we don't supply a path or parent?