clean is gone; all the intermediate files are created
in a temporary tree that is wiped when the command ends.

Not using go/build's Script because it is not well aligned
with this API.  The various builder methods are copied from
go/build and adapted.  Probably once we delete goinstall
we can delete the Script API too.

R=rogpeppe, adg, adg
CC=golang-dev
https://golang.org/cl/5483069