pylint_plugin: Consolidate imports
pylint_plugin: Consolidate imports

These are all directly importable from the `astroid` namespace, with
astroid 1.6.0. Presumably the reason I had done it this way was because
I learned/discovered the astroid interface through experimentation in
ipython.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester