fix: PREFIX_PATH not MODULE_PATH

[henryiii]

Description

Quick naming update - this actually has to set PREFIX_PATH, not MODULE_PATH (my instructions were right, but I hadn't noticed I forgot to change the name). Suggested usage:

module_dirs = importlib.metadata.entry_points(group="cmake.prefix")
dirs = (importlib.resources.files(ep.load()) for ep in module_dirs)
paths_str = ";".join(str(p) for p in dirs)
cmake_arg = f"-DCMAKE_PREFIX_PATH={paths_str}"

Suggested changelog entry:

• Will update feat: add entrypoint for cmake modules dir #4258.