import-checker: add utility to examine what module is imported easily `fromlocalfunc()` uses: - `modulename` (of the target source) to compose absolute module name imported relatively from it It is assumed that `modulename` is an `dotted_name_of_path()`-ed source file, which may have `.__init__` at the end of it. This assumption makes composing `prefix` of relative name easy. - `localmods` to examine whether there is a locally defined (= Mercurial specific) module matching against the specified name It is assumed that module names not existing in `localmods` are ones of Python standard library.