Better handle imports from modules run as scripts

[jakebailey]

Right now, our import system is "global", shared from every module. However, when a file is run as a script, say python foo/bar.py, then its sys.path is prepended with the directory in which the script lives, and its effective import root is there.

We can't handle this case, and require extraPaths or PYTHONPATH in order to set an extra import root for this scenario.

In order to better handle this case, we could have some setup to have import roots per-module, but doing so would require #443/#846/#1360/etc to be fixed in order to decouple importable names from the identity of a module.

Going through some issues:

• Unresolved import when python file in subdirectory #1599 - nested folder with imports from there, but you can run python nestedFolder/a.py and it works fine.