Fast small builds

[davidmorgan]

The opposite end of the performance characteristics to #3811 is that build_runner should be fast for small builds: i.e. as well as scaling well, the entry cost should be low.

[rrousselGit]

Small builds are decent already.

IMO the only real issues are:

• developer experience around command line and other IDE thingy
• poor scaling due to cyclic implrts contently causing everything to be regenerated

[davidmorgan]

Yes, those are higher priority.

Still, I think if it's possible to turn today's 2-4s for tiny builds into 100ms that'll be a nice improvement :)