Add option for supplying stub dependencies at build time

[r-dean]

Add --dep flag for build-time stub generation with module dependencies

Currently, projects with multiple interdependent extension modules are forced to use INSTALL_TIME stub generation (or a hacky workaround), because build-time generation has no mechanism to handle modules that depend on other modules.

I generally prefer to keep as much stuff at build time as possible as I find it keeps things more modular and debuggable. For sufficiently complex projects moving everything to install time might be the way to go, but in my usage I've found that I just want to express one or two binding modules that another depends on.

This PR adds a --dep PATH=NAME flag to stubgen (and a corresponding MODULE_DEP parameter to nanobind_add_stub()) that loads a dependency module from a file path and registers it under a provided name. This allows build-time stub generation to work correctly for multi-module projects.

Example usage

nanobind_add_stub(
    child_stubs
    MODULE child_ext
    OUTPUT child_ext.pyi
    DEPENDS child_ext base_ext
    MODULE_DEP $<TARGET_FILE:base_ext>=my_package.base_ext
)

Tests

Includes test modules (test_dep_base_ext, test_dep_other_ext, test_dep_child_ext) that exercise cross-module inheritance and multiple dependencies with MODULE_DEP. All existing stub tests continue to pass.