tests: make test-check-module-imports more robust

It failed for me without this in this way:
  tests/test-commandserver.t:19: relative import of stdlib module
  tests/test-lfs-serve.t:108: relative import of stdlib module
  tests/test-lfs-serve.t:255: relative import of stdlib module
  tests/test-lfs-serve.t:362: relative import of stdlib module
  tests/test-lfs-serve.t:406: relative import of stdlib module
  tests/test-lock.py:9: imports not lexically sorted: silenttestrunner < unittest, True, True
  tests/test-lrucachedict.py:5: imports not lexically sorted: silenttestrunner < unittest, True, True
  tests/test-match.py:5: imports not lexically sorted: silenttestrunner < unittest, True, True
  tests/test-remotefilelog-datapack.py:15: imports not lexically sorted: silenttestrunner < unittest, True, True
  tests/test-remotefilelog-histpack.py:14: imports not lexically sorted: silenttestrunner < unittest, True, True
  tests/test-simplekeyvaluefile.py:4: imports not lexically sorted: silenttestrunner < unittest, True, True
  tests/test-sshserver.py:6: imports not lexically sorted: silenttestrunner < unittest, True, True

This is because every module is considered a stdlib module, because
the stdlib_prefixes is /usr, and my repo is in /usr/local/home, which
means that sys.path contains a couple of
/usr/local/home/../hg/.. entries that count as "in the stdlib".
Fix this by preventing any path in sys.path that's inside the mercurial
source from being considered "in the stdlib".

Differential Revision: https://phab.mercurial-scm.org/D5294

contrib/import-checker.py

@@ -260,10 +260,12 b' def list_stdlib_modules():'
                 break
         else:
             stdlib_prefixes.add(dirname)
+    sourceroot = os.path.abspath(os.path.dirname(os.path.dirname(__file__)))
     for libpath in sys.path:
-        # We want to walk everything in sys.path that starts with
-        # something in stdlib_prefixes.
-        if not any(libpath.startswith(p) for p in stdlib_prefixes):
+        # We want to walk everything in sys.path that starts with something in
+        # stdlib_prefixes, but not directories from the hg sources.
+        if (os.path.abspath(libpath).startswith(sourceroot)
+            or not any(libpath.startswith(p) for p in stdlib_prefixes)):
             continue
         for top, dirs, files in os.walk(libpath):
             for i, d in reversed(list(enumerate(dirs))):