Build backend: Support namespace packages (#13833)
Unlike regular packages, specifying all `__init__.py` directories for a
namespace package would be very verbose There is e.g.
https://github.com/python-poetry/poetry/tree/main/src/poetry, which has
18 modules, or https://github.com/googleapis/api-common-protos which is
inconsistently nested. For both the Google Cloud SDK, there are both
packages with a single module and those with complex structures, with
many having multiple modules due to versioning through `<module>_v1`
versioning. The Azure SDK seems to use one module per package (it's not
explicitly documented but seems to follow from the process in
https://azure.github.io/azure-sdk/python_design.html#azure-sdk-distribution-packages
and
https://github.com/Azure/azure-sdk-for-python/blob/ccb0e03a3de748f3aabf44be94776ba37e55791f/doc/dev/packaging.md).
For simplicity with complex projects, we add a `namespace = true` switch
which disabled checking for an `__init__.py`. We only check that there's
no `<module_root>/<module_name>/__init__.py` and otherwise add the whole
`<module_root>/<module_name>` folder. This comes at the cost of
`namespace = true` effectively creating an opt-out from our usual checks
that allows creating an almost entirely arbitrary package.
For simple projects with only a single module, the module name can be
dotted to point to the target module, so the build still gets checked:
```toml
[tool.uv.build-backend]
module-name = "poetry.core"
```
## Alternatives
### Declare all packages
We could make `module-name` a list and allow or require declaring all
packages:
```toml
[tool.uv.build-backend]
module-name = ["cloud_sdk.service.storage", "cloud_sdk.service.storage_v1", "cloud_sdk.billing.storage"]
```
Or for Poetry:
```toml
[tool.uv.build-backend]
module-name = [
"poetry.config",
"poetry.console",
"poetry.inspection",
"poetry.installation",
"poetry.json",
"poetry.layouts",
"poetry.masonry",
"poetry.mixology",
"poetry.packages",
"poetry.plugins",
"poetry.publishing",
"poetry.puzzle",
"poetry.pyproject",
"poetry.repositories",
"poetry.toml",
"poetry.utils",
"poetry.vcs",
"poetry.version"
]
```
### Support multiple namespaces
We could also allow namespace packages with multiple root level module:
```toml
[tool.uv.build-backend]
module-name = ["cloud_sdk.my_ext", "local_sdk.my_ext"]
```
For lack of use cases, we delegate this to creating a workspace with one
package per module.
## Implementation
Due to the more complex options for the module name, I'm moving
verification on deserialization later, dropping the source span we'd get
from serde. We also don't show similarly named directories anymore.
---------
Co-authored-by: Andrew Gallant <andrew@astral.sh>
