[easybuild] non-standard location of setup.py for easyconfig

[Andreas Hilboll]

Hi EasyBuilders,

I'd like to add ESMpy to an easyconfig as a PythonPackage 
extension.

The problem is that the `setup.py` is not in the root of the 
source archive, but instead in a subfolder `src/addon/ESMPy/`.

Currently I have

       ('ESMpy', '7.1.0r', {
           'source_tmpl': 'esmf_7_1_0r_src.tar.gz',
           'source_urls': 
           ['http://www.earthsystemmodeling.org/esmf_releases/public/ESMF_7_1_0r/'],
           'checksums': 
           ['ae9a5edb8d40ae97a35cbd4bd00b77061f995c77c43d36334dbb95c18b00a889'],
       }),

in my `exts_list`.

Is there any way to add the information that EasyBuild should go 
to the correct subdirectory in the sources before executing 
`setup.py install`?

Thanks for your help!

Cheers,
 Andreas