rewrite `Build` and `Target`

[propensive]

This area of the code is a huge mess. We seem to construct about four different dependency graphs along the way, and I don't trust its performance. The whole thing needs a huge cleanup.

• Target should be an inner class of Build
• Make more use of lazy vals for delayed initialization
• Possibly create further intermediate representations of the build while the graph is being constructed
• Make better use of nesting classes to avoid repeating parameters everywhere