Generate src layout for mixed Python/Rust projects in maturin new

[messense]

Opt-in via --src option at first, maybe switching to src layout by default in the future.

Before

my-project
├── Cargo.toml
├── python
│   └── my_project
│       ├── __init__.py
│       └── bar.py
├── pyproject.toml
├── Readme.md
└── src
    └── lib.rs

After

my-project
├── Readme.md
├── pyproject.toml
├── src
│   └── my_project
│       ├── __init__.py
│       └── bar.py
└── rust
    ├── Cargo.toml
    └── src
        └── lib.rs

[messense]

maybe switching to src layout by default in the future

I'd like to listen to what users say about this before making the decision about changing default, it can stay opt-in if most people prefer Cargo tooling over Python tooling.

cc @clbarnes because of #501 (comment)

[clbarnes]

I've definitely found cargo and extensions to be much more easy, effective, and robust than any of the versioning/ releasing options I've used for python so would prefer to have the option of using it that way round if possible! AFAIK, python doesn't have any default/magic handling around a directory with the name src in the way that rust does - so using src for rust confers a hard advantage, where for python it's just a pretty soft convention. But happy to be overruled if anyone can recommend a workflow which works well!