contrib: make import-checker agree more with run-tests about heredocpy run-tests.py and import-checker.py take wholly different approaches to .t-tests, and the result was that they had different notions about when a Python heredoc had ended, leading to conflicts. This resolves part of the issue, and the rest I can deal with by tweaking the one offending test file.

contrib: add bzrlib to list of packages from which we import symbols Helps some tests pass the import checker.

tests: ensure demandimport test uses absolute_import