Change mixed rust/python template project layout

[messense]

Helps with #335 (comment)

Before:

❯ tree hello
hello
├── Cargo.toml
├── hello
│   └── __init__.py
├── pyproject.toml
└── src
    └── lib.rs

After:

❯ tree hello
hello
├── Cargo.lock
├── Cargo.toml
├── pyproject.toml
├── python
│   └── hello
│       └── __init__.py
└── src
    └── lib.rs

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