tests/test-check-module-imports.t

setup: prefer using the system hg to interact with the local repository

Add a findhg() function that tries to be smarter about figuring out how to run
hg for examining the local repository.  It first tries running "hg" from the
user's PATH, with the default HGRCPATH settings intact, but with HGPLAIN
enabled.  This will generally use the same version of mercurial and the same
settings used to originally clone the repository, and should have a higher
chance of working successfully than trying to run the hg script from the local
repository.  If that fails findhg() falls back to the existing behavior of
running the local hg script.

#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.

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