Only remove used modules instead of clearing sys.modules

As added to documentation [1], delete essential items from sys.modules may
cause Python to fail. Import machinery is complicated [2] and not well
documented before Python 3.3 [3] and the import system rework.

In our case, at least two issues was caused by clearing sys.modules.
- The first one impact import mechanism in our libs. If a lib import a function
(or other) with the 'from module import function' syntax, this function will be loaded,
but the module will not be properly loaded, this will result to TypeError.
For example if in our lib we use:

>>> from re import search
>>> search('(?<=-)\w+', 'spam-egg')

This will raise the following error:

  File "/usr/lib/python2.7/re.py", line 142, in search
    return _compile(pattern, flags).search(string)
TypeError: 'NoneType' object is not callable

- The second one is raised by importing a gzipped tarball in a script and store
them in 'namespace', namespace.tar for example. If we try to extract a file
from namespace.tar in subsequent scripts, we've got this error:

  File "/usr/lib/python2.7/tarfile.py", line 2112, in extractfile
    tarinfo = self.getmember(member)
  File "/usr/lib/python2.7/tarfile.py", line 1794, in getmember
    tarinfo = self._getmember(name)
  File "/usr/lib/python2.7/tarfile.py", line 2357, in _getmember
    members = self.getmembers()
  File "/usr/lib/python2.7/tarfile.py", line 1805, in getmembers
    self._load()        # all members, we first have to
  File "/usr/lib/python2.7/tarfile.py", line 2380, in _load
    tarinfo = self.next()
  File "/usr/lib/python2.7/tarfile.py", line 2319, in next
    tarinfo = self.tarinfo.fromtarfile(self)
  File "/usr/lib/python2.7/tarfile.py", line 1239, in fromtarfile
    buf = tarfile.fileobj.read(BLOCKSIZE)
  File "/usr/lib/python2.7/gzip.py", line 240, in read
    raise IOError(errno.EBADF, "read() on write-only GzipFile object")
IOError: [Errno 9] read() on write-only GzipFile object

[1] http://hg.python.org/cpython/rev/4f8160e45cb7
[2] http://python-notes.boredomandlaziness.org/en/latest/python_concepts/import_traps.html
[3] http://docs.python.org/dev/reference/import.html