refactor: move gclient-specific code to gclient dir

src/bin/gclient.rs

src/gclient/cloner.rs

src/gclient/deps_parser.rs

src/gclient/dotgclient.rs

src/gclient/gn_args.rs

src/gclient/mod.rs

src/gclient/str_to_bool_eval.py

src/gclient/var_function.py

src/gclient/var_utils.rs

src/lib.rs