Eliminate the local executeInstallPlan. The main change is that the new
execute returns BuildResults instead of an upated InstallPlan. This has
a few knock-on conseuqnces for the code that looks at the result of the
build execution.

Currently we don't actually do that much with the results in the
new-build code path (though we should) so there's less disruption than
one might imagine. The biggest change is in the integration tests which
do inspect the execution result to check things worked or didn't work as
expected.

The equivalent change for the old build code path will be more
disruptive since it does a lot of stuff with the execution results.