src: minor c++ refactors to module_wrap

• Move module_map to Environment instead of having it be global state
• std::map → std::unordered_map
• Remove one level of indirection for the map values
• Clean up empty vectors in module_map
• Call Reset() on all persistent handles in resolve_cache_
• Add a missing HandleScope to ModuleWrap::~ModuleWrap()

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

src/module_wrap