import-checker: exclude mercurial packages installed into the system path
If mercurial was installed into a directory other than the site-packages,
test-module-imports.t failed as 'mercurial.node' was listed in stdlib_modules:
testpackage/latesymbolimport.py relative import of stdlib module
Instead, we should exclude our packages explicitly.
import-checker: recurse into subtree of sys.path only if __init__.py exists
We can't assume that the site-packages is the only directory that has Python
files but is not handled as a package. For example, we have dist-packages
directory on Debian.