Import-boundary checker misses importlib.import_module, __import__ and lazy __getattr__ exports #1912

From the #1874 review. The checker misses importlib.import_module and import with a literal string, import x.y statements, and _LAZY_EXPORTS with import_module inside a package getattr. A literal import_module of a forbidden module planted in core passed. Nothing escapes today, but close it: resolve literal dynamic imports and lazy-export tables, and add the reviewer's planted-import probe as a test.