check-module-imports: ignore non-stdlib module installed by distribution

Previously, the check script would detect breezy as part of the stdlib if
installed using the debian package manager.

This silence the following complains:

tests/test-convert-bzr.t:117: imports not lexically sorted: breezy.bzr.bzrdir < sys
tests/test-convert-bzr.t:117: stdlib import "breezy.bzr.bzrdir" follows local import: breezy
tests/test-convert-bzr-ghosts.t:7: imports not lexically sorted: breezy.bzr.bzrdir < sys
tests/test-convert-bzr-ghosts.t:7: stdlib import "breezy.bzr.bzrdir" follows local import: breezy
tests/test-convert-bzr-treeroot.t:7: imports not lexically sorted: breezy.bzr.bzrdir < sys
tests/test-convert-bzr-treeroot.t:7: stdlib import "breezy.bzr.bzrdir" follows local import: breezy

Differential Revision: https://phab.mercurial-scm.org/D11249

contrib/import-checker.py

@@ -278,6 +278,8 b' def list_stdlib_modules():'
         ):
             continue
         for top, dirs, files in os.walk(libpath):
+            if 'dist-packages' in top.split(os.path.sep):
+                continue
             for i, d in reversed(list(enumerate(dirs))):
                 if (
                     not os.path.exists(os.path.join(top, d, '__init__.py'))