mypy bazel rules appear to be missing some dependencies

[laurit17]

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. `

[jhance]

Our mypy rules don't seem like they actually function here, and its not caught in our internal smoketest since it doesn't include any references to dbx_mypy_test. I'm not sure what the scope of getting this working is, but it appears a bit nontrivial (for example, we are missing //thirdparty/mypy itself, which is pretty fatal).

There's some additional complications, right now we just vendor //thirdparty/mypy directly because our internal repository rules don't respect submodules (which mypy uses) and I'd like to clean that up before considering open sourcing that.