with-src - correct develop handling

+ python setup.py develop
...
+ test-module
()
+ test-package
()
+ python -c 'import some_test_junk'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: No module named some_test_junk
+ true
+ python -c 'import setup'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: No module named setup

Perfect - we don't have junk on the import path.

no-src - incorrect develop handling

+ python setup.py develop
...
+ test-module
()
+ test-package
()
+ python -c 'import some_test_junk'
This junk should not be importable !
+ python -c 'import setup'
usage: -c [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
   or: -c --help [cmd1 cmd2 ...]
   or: -c --help-commands
   or: -c cmd --help

error: no commands supplied

This is bad, setuptools just added the root of the project on sys.path. All the junk that would never go in site-packages (when you install the distribution) is now importable ...