support for implicit namespace packages

[krpatter-intc]

There is a check for requiring an __init__.py file in the levels of the python source directory. Can we get that check removed so that maturin can be used as part of packages implementing implicit namespaces ? PEP420

maturin/src/project_layout.rs

Line 384 in e378ddd

bail!("Found a directory with the module name ({}) next to Cargo.toml, which indicates a mixed python/rust project, but the directory didn't contain an __init__.py file.", module_name)

[krpatter-intc]

After further investigation users who need namespace packages can still use the older "pkgutil" trick kf really needed:

__path__ = __import__('pkgutil').extend_path(__path__, __name__)

And so while support for implicit namespaces might be nice, its definitely not required for general namespace package usage

[konstin]

This is duplicate of #811

[krpatter-intc]

Ok, will close the PR...so the answer really is to just switch to setuptools-rust ?

The idea of using the init.py is problematic due to some downstream tools repackaging python packages in to debian flows (that complain about the init.py being there in multiple packages)

[konstin]

i'll leave the final decision on this to @messense since he's way more active than me nowadays

[Marlin-Na]

+1 for this. If the sole reason for not supporting implicit namespace is for linting agaist an opinionated best practice, can we change that to a warning instead of error?