In synchronous mode it was impossible to get the build subroutine to
attempt to build a module which had no source update, as the updater
code would filter out its build phase *in addition* to marking it as
up-to-date.

This never broke the async code as filtering out the build phase was
always unimplemented (which is what led to a specific IPC type for "code
up-to-date" in the first place).