stage: make `source_path` available before stage is built

- `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

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