The `__future__` we relied on is now, where the 3 specific things are
all included [since Python 3.0](https://docs.python.org/3/library/__future__.html):
* absolute_import
* print_function
* unicode_literals
* division
These import statements are no-ops and are no longer necessary.
* Mention utils/python_lint.py in docs/ContinuousIntegration.md
* Improve error messages in utils/python_lint.py
* Make python_lint.py fail if required modules are not found
Previously, it returned 0 in such a case, which is considered as a
successful exit.
* Continue returning success when missing modules [python_lint.py]
Reverts ffd3b7832f
In the past if one wanted to be able to develop using multiple swift repos, one
was often stymied by build-script always using build-script-impl from the swift
directory. This includes the case where one invokes the build-script from a
hypothetical directory called swift-2.
