Allow persistent pyembed runtime projects

[Skehmatics]

Allow the use of pyoxidizer's build system in a project created via init-rust-project.

This adds a runtime option to PythonDistribution.to_python_executable() that specifies the path of the Rust project, and defines it during init-rust-project

With this, applications that are happy to continue utilizing pyoxidizer's python resource discovery system can continue doing so while still allowing custom Rust code to be included in the final executable.

Closes #335

[chris-laplante]

Very timely, thank you. I just started looking at PyOxidizer this week and thought I was going crazy when I couldn't get the persistent build to work.

[chris-laplante]

Commenting back to say that this patch worked fine for me. Thanks! @indygreg would love to see this get merged.