Split out the core and main modules.

- Implicitly import everything in core into every imported module.
- Test cyclic imports.

test/module/cyclic_import/a.wren

@@ -0,0 +1,9 @@
+// nontest
+IO.print("start a")
+
+var A = "a value"
+IO.print("a defined ", A)
+var B = "b".import_("B")
+IO.print("a imported ", B)
+
+IO.print("end a")