Get logical imports in "wren_modules" working.

There's a lot of changes here and surely some rough edges to iron out.
Also, I need to update the docs. But I want to get closer to landing
this so I can build on it.

test/language/foreign/unknown_method.wren

@@ -1,3 +1,3 @@
 class Foo {
-  foreign someUnknownMethod // expect runtime error: Could not find foreign method 'someUnknownMethod' for class Foo in module 'test/language/foreign/unknown_method'.
+  foreign someUnknownMethod // expect runtime error: Could not find foreign method 'someUnknownMethod' for class Foo in module './test/language/foreign/unknown_method'.
 }