Always drop "Haskell." prefix from module names

[jespercockx]

This supersedes #297. The idea (due to @omelkonian) is that any module starting with Haskell. should be treated as defining part of the FFI with Haskell and should thus not be checked for having COMPILE pragmas, but instead be assumed to exist on the Haskell side while dropping the Haskell. prefix from the module name.

[jespercockx]

Three things to consider before merging this:

• There is a risk of introducing a name collision between Haskell.Foo.Bar and Foo.Bar. Can we do anything about this?
• Should we enforce that Haskell. modules do not produce any output?
• Can we make it clearer to the user that defining a Haskell. module is an unsafe thing to do? We can certainly add a note to the documentation, but is there anything else we can do?

[jespercockx]

Related: #316