Rename load_module.hpp to plugin_loading.hpp #913

Summary

Rename phlex/app/load_module.hpp to phlex/app/plugin_loading.hpp. The header declares loading functions for modules, resources, sources, and drivers, so its current name describes only one of its responsibilities.

Required changes

• Rename the header and update all references to it, including includes and any build or installation rules that name the file.
• Keep the existing loader declarations and behavior unchanged.

Acceptance criteria

• All source files include the new header name.
• Build and test targets that use the loader declarations continue to work.
• No references to the old header path remain.

Requested by @marcpaterno in #905 (comment).
Related PR: #905.