combine subtree merge with sparse checkout in git

I'm trying to emulate svn:externals in git. In project 'A' I need to pull down library 'B' into a sub folder of 'A'. Both A and B are separate git repos. I know I can do this with either submodules or subtree merge. However, to further complicate things, I need to pull down a subfolder of 'B' into 'A', not the whole project because it is a python module that I need to import directly and I can't have the root directory structure there or it can't be imported.

This is pretty easy to do with svn:externals. With git it seems complicated or even impossible.

Here is an example:

Project B:

-B
  - src
    __init__.py
 - test
   ...

Project A:

- A
   - src
     A.py (imports B)
     - B (partial check of sub folder 'src' as name 'B')
        __init__.py
   - test
     ...

from combine subtree merge with sparse checkout in git