- `stage.source_path` was previously overloaded; it returned `None` if it
didn't exist and this was used by client code
- we want to be able to know the `source_path` before it's created
- make stage.source_path available before it exists.
- use a well-known stage source path name, `$stage_path/src` that is
available when `Stage` is instantiated but does not exist until it's
"expanded"
- client code can now use the variable before the stage is created.
- client code can test whether the tarball is expanded by using the new
`stage.expanded` property instead of testing whether `source_path` is
`None`
- add tests for the new source_path semantics
1842873f85