Put pyi dirs at the end of the mypy path (#81)

The order of these matter if there are potential conflicts in the
MYPYPATH. Similar to generated_dirs I think it makes sense to keep the
generated file directories after the source file directories in case
there are conflicts.

This caused failures in our project since
411f64c

Author: keith

mypy/private/mypy.bzl

@@ -144,7 +144,7 @@ def _mypy_impl(target, ctx):
 
     # types need to appear first in the mypy path since the module directories
     # are the same and mypy resolves the first ones, first.
-    mypy_path = ":".join(sorted(types) + sorted(pyi_dirs) + sorted(external_deps) + sorted(imports_dirs) + sorted(generated_dirs) + sorted(generated_imports_dirs))
+    mypy_path = ":".join(sorted(types) + sorted(external_deps) + sorted(imports_dirs) + sorted(generated_dirs) + sorted(generated_imports_dirs) + sorted(pyi_dirs))
 
     output_file = ctx.actions.declare_file(ctx.rule.attr.name + ".mypy_stdout")