Bogus error: The conclusion of a builder is a mixin whose parameters

[gares]

• we write this error even if extract-conclusion-params from the builder type fails, and it fails if
• one argument is a Pack on a subject other than the current subject, since the code of copy-pack-holes only works if there is a Pack on the subject (it should not fails but succeed doing nothing
• @pi8027 has an example where a legitimate builder has a parameter that is a Pack on a constant (and its class), I don't know why