A path source with build_dir inside the project copies itself recursively #16

fetch_code copies a path source with a plain shutil.copytree(path, code_path). When build_dir is inside the project folder, the copy includes build_dir, which contains the copy being written. It recurses (build/game-source/build/game-source/...) until the OS rejects the path.

Relevant code: inputs/code.py, lines 83–85.``

This is the natural layout for a local game: the config sits in the game's folder with path: "." and build_dir: build.

Reproduction (calling fetch_code.func directly):

1. Create game/ with pxt.json and main.ts, plus game/build/.redun-cache.db, as a previous build would leave it.
2. Call fetch_code.func('{"type": "path", "value": "<abs>/game"}', "4.0.14", "<abs>/game/build/game-source").
3. It fails with shutil.Error listing paths like game/build/game-source/build/game-source/build/game-source/....

Even without the loop (for example, build_dir in a subfolder that copytree reaches later), the source copy would include earlier build output: each website stage carries its own node_modules, about 500 MB per game. build_binary_js then copies all of it again.

Expected behavior: a path source never copies build_dir. Either skip it in the copy (copytree(..., ignore=...), possibly also built/, node_modules/ and pxt_modules/), or reject a build_dir inside the source in resolve_config_paths with a clear error.

Found while reviewing PR #7, toolchain at ce12c48.