Get logical imports in "wren_modules" working.

There's a lot of changes here and surely some rough edges to iron out.
Also, I need to update the docs. But I want to get closer to landing
this so I can build on it.

test/language/module/logical_dir/wren_modules/bar/within_bar.wren

@@ -0,0 +1,5 @@
+// nontest
+var Bar = "from bar"
+System.print("ran bar module")
+
+import "foo/within_foo"

test/language/module/logical_dir/wren_modules/foo/within_foo.wren

@@ -0,0 +1,5 @@
+// nontest
+var Foo = "from foo"
+System.print("ran foo module")
+
+import "bar/within_bar"