Add support for absolute imports

Currently only relative imports are supported for local packages, however that is definitely not the pythonic way to do things. In Python absolute imports are preferred over relative imports.
Also few things are difficult by just using relative imports:

# file: constants.py
A = "a"
B = "b"

# file: main.py
from .constants import A, B  # works fine
import .constants   # Doesn't work!

By this behaviour, Python functions app forces from ... import ... syntax which is not always preferred and many times against organization specific style guidelines.

Repro steps

Covered above.

Expected behavior

Absolute imports should be supported as per python guidelines.

Actual behavior

It fails!

Known workarounds

Use relative imports

Related information

On issue #81, relative imports are suggested.

[brettcannon]

One key point that I want to make sure gets documented is if you’re going to require __app__ as the top-level directory, then anyone who wants to run their Function code outside of the Function runtime (e.g. CI for unit tests) must keep their entire Function directory in an __app__ folder in order for those imports to resolve appropriately. IOW this will will essentially dictate the folder name that everyone’s Function-related files must be kept in if anyone chooses to use absolute imports.

[Kagigz]

@maiqbal11 no problem! I’ve created another issue: #374