The setup of the input and output converters is fairly complex but was
made harder to read by nearly-duplicating its logic for input and for
output. Break out into a separate function to make clear what parts
differ between the two and what parts don't. In the process, allow the
search for a named converter to continue past a match that doesn't
support the needed direction, on the off chance of a name collision.
Make an error message more articulate in the process.
Pick-to: 6.6 6.5
Task-number: QTBUG-111228
Change-Id: I2a9de8b406c538098076f388fc8a1980b91fe16b
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
6a7b7272cd