import-checker: check modules for pure Python build correctly Before this patch, "import-checker.py" just replaces "/" in specified filenames by ".". This makes modules for pure Python build belong to "mercurial.pure" package, and prevents "import-checker.py" from correctly checking about cyclic dependency in them. This patch discards "pure" component from fully qualified name of such modules. To avoid discarding "pure" from the module name of standard libraries unexpectedly, this patch allows "dotted_name_of_path" to discard "pure" only from Mercurial specific modules, which are specified via command line arguments.