Store collapser source in the resultset private metadata as well

Store collapser source in the resultset private metadata as well

After 05a5ca4b the deparse of the row-parser coderef is incomprehensible at
best. Add the source to the private cache structure to aid debugging if it
ever becomes necessary (fingercross it won't be). Aside from the explicit
evaluation package name, and the slight change of the return of _mk_row_parser
there are no functional changes

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