feat: allow explicit location to module

[tlambert03]

Creating a PR rather than an issue to make it easier to discuss...

In my build pipeline, I would like to be able to provide a specific file location for the module, rather than relying on import module_name. For example, I'd like to be able to do this:

pybind11-stubgen my_pkg._my_module -o src --location build/cp311/_my_module.cpython-311-darwin.so

This makes it so that your module doesn't necessarily need to be in a fully qualified and importable location just to be able to create stubs. All other semantics remain the same: MODULE_NAME refers to where the module will be importable from, but under the hood, we use importlib.util.spec_from_file_location rather than importlib.import_module.

naturally, very open to feedback :). (also: not immediately sure how to integrate this into the testing setup here)

thanks!

[VelocityRa]

Bump @sizmailov? Seems useful.