chore(common): clean up `--debug` usage in build scripts

Fixes #8373.

Note that the core functionality requested in #8373 is already in place;
this commit just does some cleanup to make this easier to maintain in
the future:

* Uses `builder_is_debug_build` function to test for debug build
  (instead of `builder_has_option --debug`, which would also work but
  using the other function avoids typo issues with the option name).

* Renames `$_builder_debug` to `$_builder_debug_internal` to reduce
  naming confusion

* Documents `$builder_debug`, `builder_is_debug_build`, and clarifies
  usage of `--debug` option.

* Logs command line for dep builds (we could hide this in the future,
  but it certainly doesn't hurt for now!)