Split out the core and main modules.

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

test/module/cyclic_import/cyclic_import.wren

@@ -0,0 +1,11 @@
+var A = "a".import_("A")
+
+// Shared module should only run once:
+// expect: start a
+// expect: a defined a value
+// expect: start b
+// expect: b defined b value
+// expect: b imported a value
+// expect: end b
+// expect: a imported b value
+// expect: end a