debugger: refactor luadebug.lua

- refactor setmetatable usage to be more idiomatic;
- get rid of hardcode of `luadebug.lua` and use DEBUGGER
  instead. It will be necessary later for controlling
  of a name displayed by shell;
- refactor colored output facilities.

NO_TEST=refactoring
NO_DOC=refactoring
NO_CHANGELOG=refactoring