module.name_mapper.extension: Not working for stub folder named "__mocks__"

I am using flow version 0.43.1

There are undocumented restrictions on naming folders which can be used to put stub files in for the option `module.name_mapper.extension`.

I had a folder named `/__mocks__` and the stubs were not found, resulting in a "required module not found" error.

Using e.g. `/flow-stubs` worked.