import-checker: parse python code from .t files

import-checker: track filenames for SyntaxErrors

import-checker: track SyntaxErrors We don't really need to report SyntaxErrors, since in theory docchecker or a test will catch them, but they happen, and we can't just have the code crash, so for now, we're reporting them.

import-checker: refactor source reading This will allow .t files to generate multiple sources. It will also allow .py doctests to generate additional sources.