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")

test/module/cyclic_import/b.wren

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

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