package/
    __init__.py
    import_me.py

import re
import sys

def hello():
    pass

import re as _re
import sys as _sys

def hello():
    pass

import re
import sys

def hello():
    sys
    print('hello')

del re
del sys

>>> import del_mod
>>> del_mod.hello()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "del_mod.py", line 5, in hello
    sys
NameError: global name 'sys' is not defined

>>> import sys
>>> import re
>>> del re
>>> sys.modules['re'] = None
>>> import re
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  ImportError: No module named re

import re
import sys

def hello():
    pass

del re
del sys