Resolve computed #include directives via SimplePreprocessor

[DrGeoff]

Summary

Fixes #8. Adds support for computed #include directives (e.g. #include XSTR(COMPILETIME_INCLUDE_FILE)) in direct headerdeps mode.

• Store include argument in PreprocessorDirective during parse_directive_struct() and add include to _extract_conditional_macros() for cache correctness
• Add resolve_computed_include() to SimplePreprocessor that strips function-like macro wrappers and expands macros
• Resolve computed includes when building active_includes in preprocessing_cache.py
• Simplify _create_include_list() in headerdeps.py to use result.active_includes (net code reduction)

Test plan

• 3 new computed include tests pass (test_computed_include_no_macro, test_computed_include_with_commandline_macro, test_computed_include_with_commandline_macro_cpp)
• All 20 headerdeps tests pass
• Full test suite passes (423 passed, 2 skipped)
• Performance profiling vs v7.0.2 shows no regression (25.6% faster overall in direct mode)