feat: pyo3 support module prefix + naming

[andyscott]

Adds support for specifying the module name + module prefix for pyo3 extensions.

[UebelAndre]

Thanks! Just one question

[UebelAndre]

Why add these attributes and not change the target name to foo/bar?

[ags-openai]

I tried that first, e.g. for string_sum:

pyo3_extension(
    name = "foo/string_sum",
    srcs = ["string_sum.rs"],
    edition = "2021",
)

Building that target yields this error:

Error in fail: Crate name 'foo/string_sum' contains invalid character(s): /

[ags-openai]

There were several quirks with getting the names to line up (pymodule <-> output binary <-> crate name) + getting the outputs in the right directory so Python registers it as nested module rather than a top level module. Adding these (hopefully simple) knobs seemed like the cleanest approach.

[UebelAndre]

I'm nervous about the amount of indirection this introduces. What is the use case here that directory structure can't be made to what you want?