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.