Name clash between input file and system file leads to incorrect dependency tree.

[rchen152]

Example:

$ cat abc.py
import abc
$ importlab --tree abc.py 
Reading 1 files
Source tree:
cycle {
    abc.py
}

If I rename the file to anything else, importlab generates the correct source tree:

Source tree:
+ foo.py
    :: abc.py
        :: _weakrefset.py

I'm running importlab-0.3.0 under Python 3.6

[martindemello]

this is further complicated by the fact that import abc in a sibling file should change its behaviour depending on whether we have an __init__.py file (in a non-package environment we should import the system abc)