Make --enable-multi-repl always use multi-repl

At the moment if you use `--enable-multi-repl` then it will only use the multi repl if you have more than one component. It would be less surprising if `--enable-multi-repl` always took the multi-repl code path when it was enabled.

Start by locating the implementation of the --enable-multi-repl option and any tests covering multi-repl selection. Check the current behavior with one and multiple components, then make the flag consistently select the multi-repl path and add or update tests demonstrating both cases.

Written by the indexing model from the issue text.