mypy bazel rules appear to be missing some dependencies

I would like to try out the mypy rule, and modified the example slightly to include a dbx_mypy_test target. However, I got the following error - `every rule of type _dbx_mypy_test implicitly depends upon the target '@dbx_build_tools//dropbox/mypy:mypy_test', but this target could not be found because of: no such package '@dbx_build_tools//dropbox/mypy': BUILD file not found in directory 'dropbox/mypy' of external repository @dbx_build_tools`

Digging through the starlark code, there appear to be various labels in `mypy.bzl` that don't actually appear in the codebase (there is no `dropbox/mypy` directory). Any advice? Let me know if you need explicit steps to repro.
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