Thanks for writing this. The changes you did against the import system broke some imports on Python 3 though:
https://travis-ci.org/giampaolo/psutil/jobs/49733189
Perhaps because you changed paths in setup.py?

Also, once you manage to fix the imports don't forget to update README.rst and CREDITS.rst (add your name).

I'm looking into the import failure on py 3.x.

Yes, the import failure is because of the different names in setup.py. For example, in __init__.py you'll have to do this:

-        import _psutil_linux
+        import psutil._psutil_linux

finally fixed the tests.

I dropped the setuptools stuff.

I made different commits today and this PR cannot be merged cleanly anymore.
I created a branch in here in which I merged my changes with yours:
https://github.com/giampaolo/psutil/tree/mindw-move_extension_2_pkg
Everything is fine if I run tests via "make test" but if I try to run them via "tox" I get import related errors (if you want to try yourself just "pip install tox" then run "tox" from the main psutil directory).
Now I remember why I didn't do this before: because I tried but I always got stuck with these kind of issues.
I've been struggling with this for hours now and I really don't know how to fix this problem. Do you have any idea?

~/svn/psutil {mindw-move_extension_2_pkg|MERGING}$ tox
GLOB sdist-make: /home/giampaolo/svn/psutil/setup.py
py26 inst-nodeps: /home/giampaolo/svn/psutil/.tox/dist/psutil-3.0.0.zip
py26 runtests: PYTHONHASHSEED='2581425950'
py26 runtests: commands[0] | py.test
============================================================================================= test session starts =============================================================================================
platform linux2 -- Python 2.6.9 -- py-1.4.26 -- pytest-2.6.4
collected 0 items / 2 errors 

=================================================================================================== ERRORS ====================================================================================================
_________________________________________________________________________________ ERROR collecting test/test_memory_leaks.py __________________________________________________________________________________
test/test_memory_leaks.py:20: in <module>
    import psutil
psutil/__init__.py:68: in <module>
    from . import _pslinux as _psplatform
psutil/_pslinux.py:26: in <module>
    from . import _psutil_linux as cext
E   ImportError: cannot import name _psutil_linux
____________________________________________________________________________________ ERROR collecting test/test_psutil.py _____________________________________________________________________________________
test/test_psutil.py:54: in <module>
    import psutil
psutil/__init__.py:68: in <module>
    from . import _pslinux as _psplatform
psutil/_pslinux.py:22: in <module>
    from . import _common
E   ImportError: cannot import name _common
=========================================================================================== 2 error in 0.33 seconds ===========================================================================================

For some reason the py modules can be imported, as in:

from . import _common

...but the C extension modules are not found:

from . import _psutil_linux

Note: travis tests pass so it seems to be a problem related with tox:
https://travis-ci.org/giampaolo/psutil/builds/49831656

OK, I solved the issue by adding "usedevelop = True" in the tox config file. I merged your PR as of
e5d0946
Thanks a lot.

Unfortunately I had to revert this change in 454b1ac because relative imports no longer works if I run the interactive interpreter from within psutil installation directory.

Why on earth would you do that? Open an interactive interpreter a package and access the package?!

And even if you must have this why revert the module move? You could have reverted only the relative imports.

Well, when I work on psutil I have a shell open in psutil root directory so it's not rare I open up a python interpreter in order to try some stuff. I also sometimes executes scripts (import psutil) from within the root directory and the same issue occurs. I also reverted the changes in setup.py because the same problem applies.