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()`

PR-URL: https://github.com/nodejs/node/pull/15515


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>