Revert "move inputs/outputs to build config"

taking a different approach. they do not belong in config because it is too
late to override them fully; inputs must have already been determined by this
point. it also makes the build know more than just itself, which doesn't make
sense here.

This reverts commit 32271df868a0d86d61d46f026e068c39348ae21f.

Conflicts:
	builder/builder_test.go
input paths are config, not property of input