[lit] Add "from .main import main" back into lit/__init__.py

A refactoring commit (cf252240) removed this line. Removing it broke installing
lit with pip and setup.py.

This adds the line back in so that we can install lit again.

For an example of how this appeared, see:

http://green.lab.llvm.org/green/job/LNT_Tests/5853/

File "/Users/buildslave/jenkins/...s/__init__.py", line 2453, in resolve
    raise ImportError(str(exc))
ImportError: 'module' object has no attribute 'main'