FIX: import cycle detection which was broken.

example_errors/import_cycle1.ucg

@@ -1 +1 @@
-import "import_cycle2.ucg" as cycle2;
+let cycle2 = import "import_cycle2.ucg";

example_errors/import_cycle2.ucg

@@ -1 +1 @@
-import "import_cycle3.ucg" as cycle3;
+let cycle3 = import "import_cycle3.ucg";

example_errors/import_cycle3.ucg

@@ -1 +1 @@
-import "import_cycle1.ucg" as cycle1;
+let cycle1 = import "import_cycle1.ucg";

src/build/mod.rs

@@ -465,6 +465,10 @@ impl<'a> FileBuilder<'a> {
             Some(v) => v.clone(),
             None => {
                 let mut b = self.clone_builder();
+                b.scope
+                    .import_stack
+                    .push(normalized.to_string_lossy().to_string());
+
                 match b.build(&normalized) {
                     Ok(_) => b.get_outputs_as_val(),
                     Err(e) => {