Allow aliasing current module

[tfausak]

Fixes #13.

This PR introduces a special alias source named _ that can be used as a stand-in for the current module. For example, this input:

{-# OPTIONS_GHC
  -fplugin=Imp
  -fplugin-opt=Imp:--alias=_:Self #-}
module Qualified.Example where
print = putStrLn . show
defaultMain = Self.print

Will produce this output:

module Qualified.Example where
print = putStrLn . show
defaultMain = Qualified.Example.print

Note that the name _ is special, but the name Self is not. You can use whatever you want, like This or Current. You can even set up multiple aliases for the current module if you want that for some reason.

---

This PR also fixes a bug where imports for the current module were incorrectly being added. Previously this input:

module Example where
undefined = Example.undefined

Would produce this output:

module Example where
import qualified Example
undefined = Example.undefined

Which is always an error since a module cannot import itself. Now no import statement will be generated in this scenario.

[tfausak]

Announcement post:
https://discourse.haskell.org/t/alias-the-current-module-with-imp/9211
https://taylor.fausak.me/2024/03/31/alias-current-module/