Relative imports!

This is a breaking change because existing imports in user Wren code
that assume the path is relative to the entrypoint file will now likely
fail.

Also, stack trace output and host API calls that take a module string
now need the resolved module string, not the short name that appears in
the import.

example/import_module/lovecraft.wren

@@ -1,4 +1,4 @@
-import "cthulu" for Cthulu
+import "./cthulu" for Cthulu
 
 class Lovecraft {
   construct new() {}