Build backend: Case sensitive module discovery (#13468)

We may run on case-sensitive file systems (Linux, generally) or on
case-insensitive file systems (Windows, generally), while modules in
Python may be lower or upper case. For robustness over filesystem
casing, we require an explicit module name for modules with upper cases.

Fixes #13419

docs/configuration/build-backend.md

@@ -40,6 +40,20 @@ command includes a copy of the build backend, so when running `uv build`, the sa
 used for the build backend as for the uv process. Other build frontends, such as `python -m build`,
 will choose the latest compatible `uv_build` version.
 
+## Modules
+
+The default module name is the package name in lower case with dots and dashes replaced by
+underscores, and the default module location is under the `src` directory, i.e., the build backend
+expects to find `src/<package_name>/__init__.py`. These defaults can be changed with the
+`module-name` and `module-root` setting. The example below expects a module in the project root with
+`PIL/__init__.py` instead:
+
+```toml
+[tool.uv.build-backend]
+module-name = "PIL"
+module-root = ""
+```
+
 ## Include and exclude configuration
 
 To select which files to include in the source distribution, uv first adds the included files and