tests/test-check-module-imports.t

packaging: replace dockerlib.sh with a Python script

I want to do some more advanced things with Docker in upcoming
commits. Trying to do that with shell scripts will be a bit too
painful for my liking. Implementing things in Python will be
vastly simpler in the long run.

This commit essentially ports dockerlib.sh to a Python script.
dockerdeb and dockerrpm have been ported to use the new hg-docker
script.

hg-docker requires Python 3. I've only tested on Python 3.5.

Unlike the local packaging scripts which may need to run on old
distros, the Docker packaging scripts don't have these constraints.
So I think it is acceptable to require Python 3.5.

As part of the transition, the Docker image tags changed slightly.
I don't think that's a big deal: the Docker image names are
effectively arbitrary and are a means to an end to achieve
running commands in Docker containers.

The code for resolving the Dockerfile content allows substituting
values passed as arguments. This will be used in a subsequent commit.

Differential Revision: https://phab.mercurial-scm.org/D3759

#require test-repo

  $ . "$TESTDIR/helpers-testrepo.sh"
  $ import_checker="$TESTDIR"/../contrib/import-checker.py

  $ cd "$TESTDIR"/..

There are a handful of cases here that require renaming a module so it
doesn't overlap with a stdlib module name. There are also some cycles
here that we should still endeavor to fix, and some cycles will be
hidden by deduplication algorithm in the cycle detector, so fixing
these may expose other cycles.

Known-bad files are excluded by -X as some of them would produce unstable
outputs, which should be fixed later.

  $ testrepohg locate 'set:**.py or grep(r"^#!.*?python")' \
  > 'tests/**.t' \
  > -X hgweb.cgi \
  > -X setup.py \
  > -X contrib/debugshell.py \
  > -X contrib/hgweb.fcgi \
  > -X contrib/packaging/hg-docker \
  > -X contrib/python-zstandard/ \
  > -X contrib/win32/hgwebdir_wsgi.py \
  > -X doc/gendoc.py \
  > -X doc/hgmanpage.py \
  > -X i18n/posplit \
  > -X mercurial/thirdparty \
  > -X tests/hypothesishelpers.py \
  > -X tests/test-check-interfaces.py \
  > -X tests/test-commit-interactive.t \
  > -X tests/test-contrib-check-code.t \
  > -X tests/test-demandimport.py \
  > -X tests/test-extension.t \
  > -X tests/test-hghave.t \
  > -X tests/test-hgweb-auth.py \
  > -X tests/test-hgweb-no-path-info.t \
  > -X tests/test-hgweb-no-request-uri.t \
  > -X tests/test-hgweb-non-interactive.t \
  > -X tests/test-hook.t \
  > -X tests/test-import.t \
  > -X tests/test-imports-checker.t \
  > -X tests/test-lock.py \
  > -X tests/test-verify-repo-operations.py \
  > | sed 's-\\-/-g' | $PYTHON "$import_checker" -