- Fix bug introduced during merge of stage refactor.
- install prefix was not created before build_environment.fork()
- build_environment.fork() calls setup_dependent_environment
- python's setup_dependent_environment can inadvertently create
the install prefix before directory_layout expects it.
- Clean up Package.do_install:
- simplify control flow: parent process now entirely responsible for
creating/destroying the install prefix. cleanup is now in one place.
- Hoisting cleanup out of the child improves nesting of try/catch in
`real_work`.
- `real_work` renamed to `build_process`
5aadb6df19