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.

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 'main'.
+  foreign someUnknownMethod // expect runtime error: Could not find foreign method 'someUnknownMethod' for class Foo in module 'test/language/foreign/unknown_method'.
 }